perf(blooms): Replace JSON lib for encoding/decoding metas

[chaudum]

What this PR does / why we need it:

A lot of CPU time is spent in decoding the meta files, which are JSON files.

See this CPU profile from a trace:

github.com/json-iterator/go is a drop-in replacement for the Go stdlib encoding/json.

Benchmark

$ benchstat old.txt new.txt 
goos: linux
goarch: amd64
pkg: github.com/grafana/loki/v3/pkg/storage/stores/shipper/bloomshipper
cpu: 11th Gen Intel(R) Core(TM) i7-1185G7 @ 3.00GHz
                                     │   old.txt    │               new.txt                │
                                     │    sec/op    │    sec/op     vs base                │
_Fetcher_processMetasCacheResponse-8   4.318µ ± 12%   2.095µ ± 49%  -51.49% (p=0.000 n=10)

                                     │   old.txt   │              new.txt               │
                                     │    B/op     │    B/op     vs base                │
_Fetcher_processMetasCacheResponse-8   1301.0 ± 0%   869.0 ± 0%  -33.21% (p=0.000 n=10)

                                     │  old.txt   │              new.txt               │
                                     │ allocs/op  │ allocs/op   vs base                │
_Fetcher_processMetasCacheResponse-8   27.00 ± 0%   15.00 ± 0%  -44.44% (p=0.000 n=10)

Checklist

• [ ] Reviewed the CONTRIBUTING.md guide (required)
• [ ] Documentation added
• [ ] Tests updated
• [ ] Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• [ ] Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• [ ] If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR