fix: stop storing TimelineMetadata in index_part.json as bytes

[koivunej]

Problem

We've stored metadata as bytes within the index_part.json for since long fixed reasons. #7693 added support for reading out normal json serialization of the TimelineMetadata.

Summary of changes

Only write TimelineMetadata as json for going forward, keeping the backwards compatibility to reading the metadata as bytes.

In documentation and naming, an effort is made to add enough warning signs around TimelineMetadata so that it will receive no changes in the future. We can just add those fields to IndexPart directly instead. Changing TimelineMetadata now carries all of the backward compatibility risks without offering any benefits over changing IndexPart, where we do have a clear strategy (soft json (de)serialization).

Builds upon #7693.

[github-actions]

3024 tests run: 2885 passed, 6 failed, 133 skipped (full report)

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Failures on Postgres 16

• test_forward_compatibility: release, debug

Failures on Postgres 15

• test_forward_compatibility: release, debug

Failures on Postgres 14

• test_forward_compatibility: release, debug

# Run all failed tests locally:
scripts/pytest -vv -n $(nproc) -k "test_forward_compatibility[release-pg14] or test_forward_compatibility[debug-pg14] or test_forward_compatibility[release-pg15] or test_forward_compatibility[debug-pg15] or test_forward_compatibility[release-pg16] or test_forward_compatibility[debug-pg16]"

Flaky tests (2)

Postgres 16

• test_pageserver_lsn_wait_error_safekeeper_stop: debug

Postgres 15

• test_partial_evict_tenant[relative_equal]: release

Test coverage report is not available

_{The comment gets automatically updated with the latest test results
48174cd at 2024-05-10T14:15:50.483Z :recycle:}

[koivunej]

Failures on Postgres 16

• test_forward_compatibility: release, debug

Failures on Postgres 15

• test_forward_compatibility: release, debug

Failures on Postgres 14

• test_forward_compatibility: release, debug

These were expected 🎉.