Element-R: Fix up message shields

[richvdh]

The current way of doing this in the rust sdk is that VerificationState.to_shield_state_* returns a ShieldState, which includes a colour (red/grey/none) and a freeform text field. The latter isn’t great for EW, because we need to maintain a list of translations. It would be preferable to return an enum of some kind. Maybe we can just use the VerificationState and cargo-cult the logic into the js-sdk.

• Define a new method CryptoApi.getShieldForEvent CryptoApi.getEncryptionInfoForEvent. It should return a colour (red/grey/none) and a reason (from an enum).

  • For legacy crypto: lift the logic out of EventTile
  • For rust crypto, pull the VerificationState back out of the decryptionMetadata, and turn it into a shield. Tests.

  Implement getEncryptionInfoForEvent and deprecate getEventEncryptionInfo matrix-org/matrix-js-sdk#3693
  Estimate: 2d

• Replace event verification logic with new code in js-sdk matrix-org/matrix-react-sdk#11528: Update EventTile to call getShieldForEvent instead of doing its own verify dance.
  Significant update to the tests for EventTile.

  Estimate: 1d

• Rust implementation of getEncryptionInfoForEvent matrix-org/matrix-js-sdk#3718: Implement in rust

• Enable cypress tests for shields with rust crypto matrix-org/matrix-react-sdk#11589 Enable cypress tests for rust

Total estimate: 3.5d, plus updates for existing events.