feat(pageserver): integrate lsn lease into synthetic size (#8220)

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.



Signed-off-by: Yuchen Liang <yuchen@neon.tech>
Co-authored-by: Joonas Koivunen <joonas@neon.tech>
Co-authored-by: Christian Schwarz <christian@neon.tech>

libs/pageserver_api/src/models.rs

@@ -228,6 +228,11 @@ pub struct TimelineCreateRequest {
     pub pg_version: Option<u32>,
 }
 
+#[derive(Serialize, Deserialize, Clone)]
+pub struct LsnLeaseRequest {
+    pub lsn: Lsn,
+}
+
 #[derive(Serialize, Deserialize)]
 pub struct TenantShardSplitRequest {
     pub new_shard_count: u8,