feat(timeline): add functions for sending messages in threads

[Johennes]

send_reply is geared towards clients that don't render threads themselves. It sends a reply and optionally forwards it onto any existing thread.

This PR adds send_thread_reply for clients that do render threads themselves. This sends a message onto a thread, regardless of whether a thread existed before, and includes the fallback for non-threaded clients only if the message is not itself a reply on the thread.

• Public API changes documented in changelogs (optional)

[codecov]

Codecov Report

Attention: Patch coverage is 39.28571% with 17 lines in your changes missing coverage. Please review.

Project coverage is 86.42%. Comparing base (a539518) to head (4883789).
Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-ui/src/timeline/mod.rs	39.28%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4819      +/-   ##
==========================================
- Coverage   86.44%   86.42%   -0.03%     
==========================================
  Files         297      297              
  Lines       34527    34548      +21     
==========================================
+ Hits        29848    29859      +11     
- Misses       4679     4689      +10     

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[bnjbvr]

Nice, this should provide the ability to start a new thread.