refactor: use Option<&mut T> instead of &mut Option<T>in a few places

[bnjbvr]

This is generally a good practice, as it makes some APIs less awkward to call (in particular, callers don't have to pass &mut None when they're not interested in the optional value).

There are a few other instances of this in the code base, but after spending a few minutes trying to move them to Option<&mut>, turns out I couldn't:

• in matrix-sdk/src/sync.rs, the option is overridden at the end of the function with a new value.
• in matrix-sdk-crypto/src/identities/manager, we're actually overriding the passed &mut Option with a new Option value.
• in matrix-sdk-common/src/linked_chunk/mod.rs, the two remaining functions using &mut Option pass it multiple times, and strangely i can't clone a Option<&mut T>; I suspect the mutability makes it impossible, because it would be another live mutable alias to the inner value.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.89%. Comparing base (61fa339) to head (1de5e76).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4702      +/-   ##
==========================================
- Coverage   85.90%   85.89%   -0.02%     
==========================================
  Files         292      292              
  Lines       33907    33907              
==========================================
- Hits        29127    29123       -4     
- Misses       4780     4784       +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.