feat(pageserver): add reldir migration configs

[skyzh]

Problem

Part of #9516 per RFC at #10412

Summary of changes

Adding the necessary config items and index_part items for the large relation count work.

[github-actions]

7348 tests run: 6968 passed, 1 failed, 379 skipped (full report)

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Failures on Postgres 16

• test_metrics_normal_work: release-x86-64

# Run all failed tests locally:
scripts/pytest -vv -n $(nproc) -k "test_metrics_normal_work[release-pg16]"

Flaky tests (4)

Postgres 17

• test_physical_replication_config_mismatch_max_locks_per_transaction: release-arm64
• test_sharding_split_compaction[compact-shard-ancestors-localonly]: debug-x86-64
• test_metrics_normal_work: debug-x86-64

Postgres 14

• test_metrics_normal_work: release-x86-64

Test coverage report is not available

_{The comment gets automatically updated with the latest test results
8b7c5d3 at 2025-01-17T18:29:58.822Z :recycle:}