This PR makes some non-domain-specific changes across multiple crates
that are required for proper testing of features implemented for #5397.
* Adds a `experimental-encrypted-state-events` feature flag across the
SDK.
* Introduces a feature set into xtask to ensure feature-gated tests are
run during CI.
* Minor fix to a test that would otherwise fail with the newly
introduced CI.
Skye Elliot
ae7f0fe022