Add more methods for converting to/from sender IDs

[swedgwood]

Including some tests for sender IDs

Signed-off-by: Sam Wedgwood <sam@wedgwood.dev>

[codecov]

Codecov Report

Patch coverage: 77.27% and project coverage change: +0.15% 🎉

Comparison is base (740f742) 58.40% compared to head (484e769) 58.56%.

❗ Current head 484e769 differs from pull request most recent head c0669e0. Consider uploading reports for the commit c0669e0 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #412      +/-   ##
==========================================
+ Coverage   58.40%   58.56%   +0.15%     
==========================================
  Files          51       51              
  Lines        7040     7062      +22     
==========================================
+ Hits         4112     4136      +24     
+ Misses       2538     2534       -4     
- Partials      390      392       +2     

Files Changed	Coverage Δ
spec/userid.go	84.61% <60.00%> (-2.62%)	⬇️
spec/senderid.go	81.25% <82.35%> (+47.91%)	⬆️

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