Replace encoding/json with json-iterator/go in backend

[zalegrala]

What this PR does:

To improve the performance of json marshaling/unmarshaling of the tenant index and the block metas, swap out the library in use.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[zalegrala]

❯ benchstat main.txt jsonIter.txt
goos: linux
goarch: amd64
pkg: github.com/grafana/tempo/tempodb/backend
cpu: AMD Ryzen 7 PRO 7840U w/ Radeon 780M Graphics
                  │  main.txt   │            jsonIter.txt             │
                  │   sec/op    │   sec/op     vs base                │
IndexUnmarshal-16   57.49µ ± 3%   41.67µ ± 2%  -27.52% (p=0.000 n=10)

                  │   main.txt   │            jsonIter.txt             │
                  │     B/op     │     B/op      vs base               │
IndexUnmarshal-16   55.71Ki ± 0%   53.02Ki ± 0%  -4.82% (p=0.000 n=10)

                  │  main.txt  │            jsonIter.txt             │
                  │ allocs/op  │ allocs/op   vs base                 │
IndexUnmarshal-16   206.0 ± 1%   421.0 ± 0%  +104.37% (p=0.000 n=10)