Add new API to request missing secrets

[BillCarsonFr]

Adds a new crypto API to request missing secrets + the ffi bindings.

This API will look at the missing private secrets (usk, ssk, msk, megolm.backup), and if they are not present on the current session it will make requests for them.
The request are properly saved in "outgoing_request store" to ensure proper handling, it's also checking if there are no existing requests in fligh.

• Public API changes documented in changelogs (optional)

Signed-off-by:

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (4478826) 77.94% compared to head (5080c54) 78.09%.
Report is 22 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2641      +/-   ##
==========================================
+ Coverage   77.94%   78.09%   +0.15%     
==========================================
  Files         190      190              
  Lines       19663    19680      +17     
==========================================
+ Hits        15327    15370      +43     
+ Misses       4336     4310      -26     

Files	Coverage Δ
crates/matrix-sdk-crypto/src/machine.rs	83.48% <100.00%> (+0.53%)	⬆️

... and 5 files with indirect coverage changes

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[poljar]

Seems sensible, not sure when/how we'll utilize this in the main crate but seems to be generally useful to have this ability.