crypto: Notify when the identity status of a user changes

[andybalaam]

Fixes element-hq/element-meta#2526

If you follow the stream returned from matrix_sdk::room::Room::subscribe_to_identity_status_changes you will receive an update every time someone in the room changes identity status e.g. becomes "Unpinned", meaning the UI should display a warning.

[codecov]

Codecov Report

Attention: Patch coverage is 54.70085% with 53 lines in your changes missing coverage. Please review.

Project coverage is 84.14%. Comparing base (2a03de3) to head (9b023af).

Files with missing lines	Patch %	Lines
...tes/matrix-sdk/src/room/identity_status_changes.rs	0.00%	26 Missing ⚠️
crates/matrix-sdk/src/room/mod.rs	0.00%	19 Missing ⚠️
...x-sdk-crypto/src/identities/room_identity_state.rs	90.90%	6 Missing ⚠️
...ates/matrix-sdk/src/encryption/identities/users.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4022      +/-   ##
==========================================
- Coverage   84.23%   84.14%   -0.10%     
==========================================
  Files         266      268       +2     
  Lines       28349    28466     +117     
==========================================
+ Hits        23881    23953      +72     
- Misses       4468     4513      +45     

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