proxy: make sql-over-http max request/response sizes configurable

[cloneable]

Problem

Summary of changes

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]

5020 tests run: 4856 passed, 0 failed, 164 skipped (full report)

---

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-x86-64

Code coverage* (full report)

• functions: 31.8% (7420 of 23314 functions)
• lines: 49.8% (59619 of 119801 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
ea915ca at 2024-09-18T11:50:47.791Z :recycle:}