pageserver: refuse to run without remote storage

[jcsp]

Problem

Since #6769, the pageserver is intentionally not usable without remote storage: it's purpose is to act as a cache to an object store, rather than as a source of truth in its own right.

Summary of changes

• Make remote storage configuration mandatory: the pageserver will refuse to start if it is not provided.

This is a precursor that will make it safe to subsequently remove all the internal Option<>s

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]

3060 tests run: 2927 passed, 0 failed, 133 skipped (full report)

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Flaky tests (1)

Postgres 14

• test_lock_time_tracing: debug

Code coverage* (full report)

• functions: 31.4% (6330 of 20161 functions)
• lines: 47.3% (47754 of 100969 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
c6cf7db at 2024-05-13T10:19:55.408Z :recycle:}