Support GetLiveFiles on secondary DB instances

[archang19]

Summary:
GetLiveFiles was previously blocked on secondary instances with
Status::NotSupported, even though the operation is safe to perform.

The only reason GetLiveFiles was originally blocked is that it defaults
to flushing the memtable (flush_memtable=true), which is a write
operation. However, the actual implementation in DBImpl::GetLiveFiles
(db_filesnapshot.cc) does two things:

1. Optionally flush the memtable — which we skip by passing
   flush_memtable=false. The secondary already overrides
   FlushForGetLiveFiles() as a no-op, so even if true were passed
   it would not actually flush.

2. Read live file state from versions_ under the mutex — this is
   purely read-only. It iterates the ColumnFamilySet to collect live
   table and blob file numbers, builds relative file paths for SST
   files, blob files, CURRENT, MANIFEST, and OPTIONS, and reads the
   manifest file size.

The secondary maintains its own VersionSet which it keeps up to date
via MANIFEST replay in TryCatchUpWithPrimary(). So all of this state
is valid and accurate — it reflects exactly which files the secondary
considers live at its current replay point.

This is the same approach used by DBImplReadOnly and CompactedDBImpl,
which both delegate to DBImpl::GetLiveFiles with flush_memtable=false.

Differential Revision: D97563143

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@archang19 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D97563143.

[github-actions]

✅ clang-tidy: No findings on changed lines

Completed in 116.0s.

[meta-codesync]

This pull request has been merged in 89e384c.