refactor(sdk): compute a push context only once when computing push actions for a bunch of events

[bnjbvr]

This has three benefits:

• we don't need to recompute the full push context (which requires a few database accesses each time) when we're calling try_decrypt_event() for lots of events (which can happen when doing /context queries).
• this makes the workflow very explicit: you may get (or not) a PushContext, and use that to compute the push actions after decrypting
• we will properly fill the push actions for non-encrypted messages received with /messages at the right time (in self.try_decrypt_event), so we don't need to do it in an extra processing step

This is a preparatory refactoring for threads, since we'll want to use similar logic when fetching the thread roots, or when back-paginating inside a thread.

Part of #4869.

[codecov]

Codecov Report

Attention: Patch coverage is 94.33962% with 3 lines in your changes missing coverage. Please review.

Project coverage is 85.98%. Comparing base (7adf60d) to head (e99e448).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
...atrix-sdk-base/src/response_processors/timeline.rs	77.77%	2 Missing ⚠️
crates/matrix-sdk-ui/src/notification_client.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4962   +/-   ##
=======================================
  Coverage   85.98%   85.98%           
=======================================
  Files         325      325           
  Lines       35660    35647   -13     
=======================================
- Hits        30663    30652   -11     
+ Misses       4997     4995    -2     

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