storagemgr: Add symbolic ref support in `ReferenceUpdate`

The `ReferenceUpdate` struct captures reference updates. This only works with regular reference updates. Similar to the previous commit, we add support for symbolic reference updates by adding `OldTarget` and `NewTarget` fields to the struct. We then ensure we propagate this data correctly.

However, since the reference transaction hook hasn't been hooked to work with symbolic updates, we shouldn't expect any symbolic updates to be propagated. In upcoming commits, we should add support within reference transaction hook and also ensure we only apply symbolic updates for the 'HEAD' ref and that too only when using the reftable backend.

Part of #5867