pageserver: use an Option for GcCutoffs::time (#11984)

## Problem

It is not currently possible to disambiguate a timeline with an
uninitialized PITR cutoff from one that was created within the PITR
window -- both of these have `GcCutoffs::time == Lsn(0)`. For billing
metrics, we need to disambiguate these to avoid accidentally billing the
entire history when a tenant is initially loaded.

Touches https://github.com/neondatabase/cloud/issues/28155.

## Summary of changes

Make `GcCutoffs::time` an `Option<Lsn>`, and only set it to `Some` when
initialized. A `pitr_interval` of 0 will yield `Some(last_record_lsn)`.

This PR takes a conservative approach, and mostly retains the old
behavior of consumers by using `unwrap_or_default()` to yield 0 when
uninitialized, to avoid accidentally introducing bugs -- except in cases
where there is high confidence that the change is beneficial (e.g. for
the `pageserver_pitr_history_size` Prometheus metric and to return early
during GC).

pageserver/src/http/routes.rs

@@ -449,7 +449,7 @@ async fn build_timeline_info_common(
     // Internally we distinguish between the planned GC cutoff (PITR point) and the "applied" GC cutoff (where we
     // actually trimmed data to), which can pass each other when PITR is changed.
     let min_readable_lsn = std::cmp::max(
-        timeline.get_gc_cutoff_lsn(),
+        timeline.get_gc_cutoff_lsn().unwrap_or_default(),
         *timeline.get_applied_gc_cutoff_lsn(),
     );