Add event.internal_metadata.instance_name (#17300)

Add `event.internal_metadata.instance_name` (the worker instance that persisted the event) to go alongside the existing `event.internal_metadata.stream_ordering`.

`instance_name` is useful to properly compare and query for events with a token since you need to compare both the `stream_ordering` and `instance_name` against the vector clock/`instance_map` in the `RoomStreamToken`.

This is pre-requisite work and may be used in https://github.com/element-hq/synapse/pull/17293

Adding `event.internal_metadata.instance_name` was first mentioned in the initial Sliding Sync PR while pairing with @erikjohnston, see 09609cb0db (diff-5cd773fb307aa754bd3948871ba118b1ef0303f4d72d42a2d21e38242bf4e096R405-R410)

rust/src/events/internal_metadata.rs

@@ -204,6 +204,8 @@ pub struct EventInternalMetadata {
     /// The stream ordering of this event. None, until it has been persisted.
     #[pyo3(get, set)]
     stream_ordering: Option<NonZeroI64>,
+    #[pyo3(get, set)]
+    instance_name: Option<String>,
 
     /// whether this event is an outlier (ie, whether we have the state at that
     /// point in the DAG)
@@ -232,6 +234,7 @@ impl EventInternalMetadata {
         Ok(EventInternalMetadata {
             data,
             stream_ordering: None,
+            instance_name: None,
             outlier: false,
         })
     }