Avoid re-calculating bundled aggregations for cached results

[clokep]

Related to #11583 -- currently when we generate a sync response we cache it for a period of time (as clients sometimes timeout and retry). Unfortunately the bundled aggregations are added during the serialization step, which occurs after caching, and is re-done even when using a cached response.

Ideally we should be calculating the bundled aggregations earlier in a different manner and the serialization methods should be synchronous.

See https://matrix.to/#/!XaqDhxuTIlvldquJaV:matrix.org/$1E_K64Dd5p6iFMS4agak62bn5JZl3Ry-6XiTXYzOmhU?via=matrix.org&via=vector.modular.im&via=envs.net for a brief conversation around this.

[clokep]

It might make more sense to:

• Explicitly calculate the bundled aggregations at some point by calling a method on a list of events.
• The bundled aggregations could be store on _EventInternalMetadata (similar to how redacted is handled).
• The event serialization code would then have everything necessary to go from EventBase -> JSON.

It would also be slightly less magical, but might be a bit more verbose.

[clokep]

My hope with the above is that we can avoid this problem for other APIs too, if it exists. Note that we need to be careful though since events get cached and modifying them will modify a shared copy.

[clokep]

Oops, this isn't "done" until #11659 lands.