feat(sdk): Add day dividers to the experimental timeline

[zecakeh]

Tested locally in Fractal.

[codecov]

Codecov Report

Base: 74.21% // Head: 74.18% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (7c60493) compared to base (ee713d4).
Patch coverage: 68.42% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1251      +/-   ##
==========================================
- Coverage   74.21%   74.18%   -0.04%     
==========================================
  Files         114      115       +1     
  Lines       13006    13060      +54     
==========================================
+ Hits         9653     9689      +36     
- Misses       3353     3371      +18     

Impacted Files	Coverage Δ
...ates/matrix-sdk/src/room/timeline/event_handler.rs	70.47% <67.85%> (-1.08%)	⬇️
...rates/matrix-sdk/src/room/timeline/virtual_item.rs	100.00% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[jplatte]

Looks pretty good, just a few small questions and suggestions.