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