Embedded calls should prefer homeserver `matrix_rtc_session` settings over defaults

### Your use case

#### What would you like to do?

Currently, calls using the embedded version of Element Call use the default set of options for `matrix_rtc_session`. This means that if your homeserver is slow or prone to restarts, you cannot adjust the rates at which you send or expire delayed events locally.

#### Why would you like to do it?

Homeservers have different implementations, exist in different environments, and have different uptimes / downtimes. It is not sensible to have a single config to apply to all of these, but to have configuration that applies based on the specifics of the homeserver.

#### How would you like to achieve it?

The most sensible way is probably extending the [MatrixRTC spec](https://github.com/matrix-org/matrix-spec-proposals/blob/toger5/matrixRTC/proposals/4143-matrix-rtc.md#discovery-of-rtc-transports) to include sensible configuration. However, given this is Element Call specific, it might just have to exist as well-known values for the moment.

### Have you considered any alternatives?

Simply bumping the defaults to cover all bases, although this has the dual problems of:

1. It may not be enough in heavily constrained environments.
2. This lowers the responsiveness of the application, as genuine disconnects go unnoticed for longer.

### Additional context

_No response_