fix(matrix): add missing txid to send message call

[piksel]

The Matrix Client/Server spec requires that the send message endpoint includes a unique transaction id with each request. The matrix service omits this id for some reason, and the original testing server probably allowed this.

This PR adds an incrementing counter as the transaction ID, which should allow it to work with conduit matrix server.

Fixes #415.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.24%. Comparing base (3428e5e) to head (fe44270).
⚠️ Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/services/matrix/matrix_client.go	73.91%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #416      +/-   ##
==========================================
+ Coverage   79.12%   79.24%   +0.11%     
==========================================
  Files         102      102              
  Lines        4488     4480       -8     
==========================================
- Hits         3551     3550       -1     
+ Misses        758      752       -6     
+ Partials      179      178       -1     

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[katiekloss]

Is there anything I can do to help get this over the finish line? More code coverage? It looks like most of the uncovered paths are for handling errors.