storcon: improve initial shard scheduling

[VladLazar]

Problem

Scheduling on tenant creation uses different heuristics compared to the scheduling done during
background optimizations. This results in scenarios where shards are created and then immediately
migrated by the optimizer.

Summary of changes

1. Make scheduler aware of the type of the shard it is scheduling (attached vs secondary).
   We wish to have different heuristics.
2. For attached shards, include the attached shard count from the context in the node score
   calculation. This brings initial shard scheduling in line with what the optimization passes do.
3. Add a test for (2).

This looks like a bigger change than required, but the refactoring serves as the basis for az-aware
shard scheduling where we also need to make the distinction between attached and secondary shards.

Closes #8969

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]

5133 tests run: 4969 passed, 0 failed, 164 skipped (full report)

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Flaky tests (5)

Postgres 17

• test_pageserver_compaction_smoke: release-arm64
• test_neon_cli_basics: release-arm64
• test_subscriber_restart: debug-x86-64

Postgres 16

• test_subscriber_restart: release-x86-64

Postgres 14

• test_ondemand_wal_download_in_replication_slot_funcs: release-x86-64

Code coverage* (full report)

• functions: 32.1% (7456 of 23228 functions)
• lines: 49.9% (60041 of 120270 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
7825755 at 2024-09-24T09:14:55.289Z :recycle:}

[jcsp]

I like how this is expressed with types. Use of member order for the contributions to score is much more readable than my original .0, .1, .2 line of code.