Add Timeline::retry_send

[jplatte]

Previously a pretty obvious hole in the timeline API: When sending failed, there was no way to deal with the failed local echo. This adds one way to do that, we should also add the ability to cancel a failed local echo (in a separate PR).

[codecov]

Codecov Report

Patch coverage: 75.55% and project coverage change: +0.02 🎉

Comparison is base (407375a) 75.22% compared to head (eb312f4) 75.24%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2022      +/-   ##
==========================================
+ Coverage   75.22%   75.24%   +0.02%     
==========================================
  Files         147      147              
  Lines       16034    16076      +42     
==========================================
+ Hits        12062    12097      +35     
- Misses       3972     3979       +7     

Impacted Files	Coverage Δ
crates/matrix-sdk-ui/src/timeline/mod.rs	84.91% <68.00%> (-2.19%)	⬇️
...s/matrix-sdk-ui/src/timeline/event_item/content.rs	56.57% <75.00%> (+0.49%)	⬆️
crates/matrix-sdk-ui/src/timeline/inner.rs	68.31% <86.66%> (+0.64%)	⬆️
crates/matrix-sdk/src/sliding_sync/list/mod.rs	92.75% <100.00%> (-0.04%)	⬇️

... and 1 file with indirect coverage changes

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

[Hywan]

Good work, thanks!

I wish we could have a test to ensure this behavior. How hard would it be?

[jplatte]

I guess testing shouldn't be that hard if we use HTTP mocking to make the first try fail. I'll give it a try.