Add webp support to px.imshow

[maximsmol]

WebP images are supported by all browsers (considered "Baseline" by MDN, see https://caniuse.com/webp)

They are typically higher quality and smaller than corresponding PNG and JPEG images:

• https://developers.google.com/speed/webp/docs/webp_study
• https://developers.google.com/speed/webp/docs/webp_lossless_alpha_study

This PR adds support for webp as a binary_format in imshow and adds a way to pass parameters to the backend (Pillow) to allow full configuration of the encoder e.g. choosing between lossless and lossy WebPs, adjusting the speed/size tradeoff.

Documentation PR

• I've seen the doc/README.md file
• This change runs in the current version of Plotly on PyPI and targets the doc-prod branch OR it targets the master branch
• If this PR modifies the first example in a page or adds a new one, it is a px example if at all possible
• Every new/modified example has a descriptive title and motivating sentence or paragraph
• Every new/modified example is independently runnable
• Every new/modified example is optimized for short line count and focuses on the Plotly/visualization-related aspects of the example rather than the computation required to produce the data being visualized
• Meaningful/relatable datasets are used for all new examples instead of randomly-generated data where possible
• The random seed is set if using randomly-generated data in new/modified examples
• New/modified remote datasets are loaded from https://plotly.github.io/datasets and added to https://github.com/plotly/datasets
• Large computations are avoided in the new/modified examples in favour of loading remote datasets that represent the output of such computations
• Imports are plotly.graph_objects as go / plotly.express as px / plotly.io as pio
• Data frames are always called df
• fig = <something> call is high up in each new/modified example (either px.<something> or make_subplots or go.Figure)
• Liberal use is made of fig.add_* and fig.update_* rather than go.Figure(data=..., layout=...) in every new/modified example
• Specific adders and updaters like fig.add_shape and fig.update_xaxes are used instead of big fig.update_layout calls in every new/modified example
• fig.show() is at the end of each new/modified example
• plotly.plot() and plotly.iplot() are not used in any new/modified example
• Hex codes for colors are not used in any new/modified example in favour of these nice ones

Code PR

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the codegen files and not generated files.
• I have added tests (if submitting a new feature or correcting a bug) or
  modified existing tests.
• For a new feature, I have added documentation examples in an existing or
  new tutorial notebook (please see the doc checklist as well).
• I have added a CHANGELOG entry if fixing/changing/adding anything substantial.
• For a new feature or a change in behaviour, I have updated the relevant docstrings in the code to describe the feature or behaviour (please see the doc checklist as well).

[maximsmol]

I am aware builds are failing but I don't think these failures are related to my changes. Please let me know if I'm missing something in my patch that caused this and I'd be happy to fix the issues.

Failures are:

1. python_38_orca with

<snip>
plotly/tests/test_orca/test_to_image.py 
Too long with no output (exceeded 20m0s): context deadline exceeded

2. plotly_js_dev_build with

plotly/tests/test_core/test_figure_messages/test_on_change.py ........F. [ 10%]
<snip>
plotly/tests/test_core/test_graph_objs/test_figure.py FFFFFFFFFFFFFFFF.  [ 27%]
plotly/tests/test_core/test_graph_objs/test_figure_properties.py FFFFFFF [ 29%]
FFFFFFFFFFFFFFFFFFFF                                                     [ 34%]
<snip>
plotly/tests/test_core/test_graph_objs/test_graph_objs.py F.F......      [ 36%]
<snip>
plotly/tests/test_core/test_graph_objs/test_layout_subplots.py FFFFFFFFF [ 39%]
FF                                                                       [ 39%]
<snip>
plotly/tests/test_core/test_graph_objs/test_property_assignment.py F.... [ 43%]
<snip>
plotly/tests/test_core/test_graph_objs/test_template.py FFFFFFFFFFFFFFFF [ 52%]
FFFFFFF......F.                                                          [ 56%]
plotly/tests/test_core/test_graph_objs/test_to_ordered_dict.py FF        [ 56%]
<snip>
plotly/tests/test_core/test_offline/test_offline.py FFFFFFFFFFFFFFFFFFFF [ 63%]
FFFF                                                                     [ 64%]
<snip>
Did you mean "heatmap"?

Bad property path:
heatmapgl
^^^^^^^^^
=== 108 failed, 338 passed, 1 skipped, 1 deselected, 521 warnings in 41.11s ====

The highlighted error message (about heatmapgl) seems to be the cause of most or all of these though it's hard to verify because there are 35000 lines of output.