timeline: include the remote event's origin in the timeline position/end

[bnjbvr]

And don't assume inserting to the end means it's coming from sync — as it won't be true for forward pagination anymore.

Supports #3234, extracted from #3329.

[codecov]

Codecov Report

Attention: Patch coverage is 84.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 83.61%. Comparing base (d986437) to head (fa4018b).
Report is 1 commits behind head on main.

Files	Patch %	Lines
crates/matrix-sdk-ui/src/timeline/inner/state.rs	62.50%	3 Missing ⚠️
crates/matrix-sdk-ui/src/timeline/event_handler.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3350      +/-   ##
==========================================
- Coverage   83.62%   83.61%   -0.01%     
==========================================
  Files         241      241              
  Lines       24916    24926      +10     
==========================================
+ Hits        20835    20842       +7     
- Misses       4081     4084       +3     

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[andybalaam]

Thanks!