Cherry-pick recent changes from master - #9240

See individual commits for details.

Commit #1
=========

transport: Enforce overall limit on controlBuffer size

This change introduces an overall limit on the number of items in the
controlBuffer. Once this limit is reached, reading from the wire is
stopped to give the transport enough time to process items in the
controlBuffer.

HEADERS and DATA frames don't count towards the throttling limit.

Earlier, throttling was only applied to frames that were considered to
result in control frames being written on the transport. But this was
found insufficient in some cases involving RapidReset and MadeYouReset.

Other changes:
* Split the client and server headers into separate structs
* Increase the throttling limit to 100 (since more items are being
  throttled now)

Commit #2
=========

xds/rbac: Fix multiple deviations in implementation from A41 spec

Summary of changes:
* Handle Metadata and RequestedServerName permissions.
* Fix panic in xDS RBAC engine when parsing unsupported fields in
  NotRule/NotId permissions in matchers.go.
* Support the deprecated `source_ip` principal identifier by
  treating it as equivalent to `direct_remote_ip`.