feat(pageserver): store reldir in sparse keyspace

[skyzh]

Problem

Part of #9516

Summary of changes

This patch adds the support for storing reldir in the sparse keyspace. All logic are guarded with the rel_size_v2_enabled flag, so if it's set to false, the code path is exactly the same as what's currently in prod.

Note that we did not persist the rel_size_v2_enabled flag and the logic around it will be implemented in the next patch. (i.e., what if we enabled it, restart the pageserver, and then it gets set to false? we should still read from v2 using the rel_size_v2_migration_status in the index_part).

I also refactored the metrics so that it can work with the new reldir store. However, this metric is not correctly computed for reldirs (see the comments) before. With the refactor, the value will be computed only when we have an initial value for the reldir size. The refactor keeps the incorrectness of the computation when there are more than 1 database.

[github-actions]

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_{The comment gets automatically updated with the latest test results
8bb1147 at 2025-01-30T23:04:26.610Z :recycle:}