fix(pageserver): is_archived should be optional

[skyzh]

Problem

Summary of changes

Set the field to optional, otherwise there will be decode errors when newer version of the storage controller receives the JSON from older version of the pageservers.

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

[bayandin]

Testing for such cases will be added in https://github.com/neondatabase/cloud/issues/14722

[github-actions]

3815 tests run: 3709 passed, 0 failed, 106 skipped (full report)

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Code coverage* (full report)

• functions: 32.5% (7424 of 22870 functions)
• lines: 50.6% (60118 of 118869 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
aca8897 at 2024-09-03T17:16:13.565Z :recycle:}