Add method for checking whether our other devices are cross-signed, even when this device isn't

[duxovni]

This is something I'm now doing multiple different places in matrix-react-sdk, so I figured it was worth adding as a method on MatrixClient.

Notes: Add MatrixClient.checkIfOwnDeviceCrossSigned method, for checking if another of our devices is cross-signed regardless of whether this device trusts the MSK.

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This PR currently has no changelog labels, so will not be included in changelogs.

A reviewer can add one of: T-Deprecation, T-Enhancement, T-Defect, T-Task to indicate what type of change this is, or add Type: [enhancement/defect/task] to the description and I'll add them for you.

[codecov-commenter]

Codecov Report

Merging #2288 (62d2d0e) into develop (f963fea) will increase coverage by 0.01%.
The diff coverage is 87.50%.

@@             Coverage Diff             @@
##           develop    #2288      +/-   ##
===========================================
+ Coverage    59.52%   59.54%   +0.01%     
===========================================
  Files           91       91              
  Lines        16412    16420       +8     
  Branches      3782     3783       +1     
===========================================
+ Hits          9770     9777       +7     
- Misses        6642     6643       +1     

Impacted Files	Coverage Δ
src/client.ts	37.82% <75.00%> (+0.06%)	⬆️
src/crypto/index.ts	64.43% <100.00%> (+0.10%)	⬆️