chore(data model): Add metadata support for end-to-end acknowledgements (#7141)

* Drop unreferenced struct from RFC

* chore(data model): Add metadata for event finalization

* Add the event finalizers to the top-level metadata

* Move cloning into registration functions

This avoids extraneous use of `Arc::clone` when the item is already in
the table.

* Add a "counting" DashMap to provide serialization semantics

* Back out support for serialzing finalization status

In discussion with Brian and Luke, we decided to punt for now on
supporting end-to-end acknowledgements after an event has been buffered
to disk. This greatly simplifies the initial requirements, but we will
come back to this later.

* Drop remaining `impl From<LogEvent>` that silently drop metadata

* Quiet a clippy warning

In `enum BufferInputCloner`, the `Disk` variant is quite a bit larger
than `Memory` with the increased size of `Event` (233 vs 25 bytes).
Clippy recommends boxing this, but that is beyond the scope of this PR.

* Add `metadata_mut` accessors to event types

* Rename `EventStatus::NoOp` to `Recorded`

Signed-off-by: Bruce Guenter <bruce.guenter@datadoghq.com>