pageserver: provide more context in GCResult and clarify timeline_gc caller intent.

[yliang412]

Adapted from #9024 (comment):

We have many tests where we do manual PageserverHttpClien.timeline_gc and expect GC not to be skipped or some layer is removed. It is possible to query the returned GcResult to learn about whether some layers are removed, but we don't have enough information to know whether GC is skipped.

We also want to differentiate "skipping GC" from "doing GC but removed 0 layers" since during a GC iteration we refresh per timeline GCInfo even if we removed no layers.

We should provide more information (maybe the gc blocking reasons) in the return value and use assertion on Python side.