proxy: refactor json parsing

[conradludgate]

Problem

current json parsing is a little bit expensive with untagged enums and intermediate allocations

Summary of changes

write custom deserialize impls to avoid untagged enum cost, and try and remove some intermediate allocs.

I eventually want to move to using DeserializeSeed to use an arena and to remove the need of serde_json::Value during parsing entirely.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

4968 tests run: 4804 passed, 0 failed, 164 skipped (full report)

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Flaky tests (4)

Postgres 17

• test_pageserver_compaction_smoke: release-arm64
• test_scrubber_physical_gc[4]: debug-x86-64, release-arm64

Postgres 16

• test_subscriber_restart: release-arm64

Code coverage* (full report)

• functions: 32.0% (7414 of 23193 functions)
• lines: 49.6% (59563 of 120028 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
98393c4 at 2024-09-17T11:38:38.076Z :recycle:}