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

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (3428e5e) 79.12% compared to head (fe44270) 79.24%.

Files	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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