feat(pageserver): integrate lsn lease into synthetic size

[yliang412]

Part of #7497, closes #8071. (accidentally closed #8208, reopened here)

Problem

After the changes in #8084, we need synthetic size to also account for leased LSNs so that users do not get free retention by running a small ephemeral endpoint for a long time.

Summary of changes

This PR integrates LSN leases into the synthetic size calculation. We model leases as read-only branches started at the leased LSN (except it does not have a timeline id).

Other changes:

• Add new unit tests testing whether a lease behaves like a read-only branch.
• Change /size_debug response to include lease point in the SVG visualization.
• Fix /lsn_lease HTTP API to do proper parsing for POST.

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

[yliang412]

Leases vs. Equivalent RO Timeline

Leases

RO Timeline

[github-actions]

3018 tests run: 2903 passed, 0 failed, 115 skipped (full report)

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Code coverage* (full report)

• functions: 32.6% (6929 of 21236 functions)
• lines: 50.0% (54437 of 108800 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
9f4ffb3 at 2024-07-04T15:05:17.367Z :recycle:}

[koivunej]

With the added documentation breadcrumbs this is good.

[problame]

Regarding the svg dra changes: why not store the bool (or even better, an enum BranchKind { Branch, Lease }) in SvgDraw::branches instead of for each segment?
Feels more natural there to me.

(The enum instead of bool is a usual readability pet peeve of mine, kind of orthogonal to where we store the information)

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The insert_and_create_ro_branch and insert_and_acquire_lease look almost identical.

For clarity, I think it makes sense to DRY by parametrizing them with "lease" | "branch".

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This looks like what we discussed.
Have some minor nits, but, good job!