ffi: Expose encryption settings via FFI

[andybalaam]

Fixes #3130

This allows providing encryption settings while constructing a client via FFI, instead of hard-coding specific values.

We do it by holding on to an EncryptionSettings inside ClientBuilder and updating it when certain methods on the builder are called. During build_inner() we provide those EncryptionSettings by calling with_encryption_settings on inner_builder.

I added an enum BackupDownloadStrategy to be passed as an argument to backup_download_strategy. Let me know if there's a better way.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.67%. Comparing base (7c68096) to head (ebcf1c4).
Report is 38 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3326      +/-   ##
==========================================
+ Coverage   83.65%   83.67%   +0.01%     
==========================================
  Files         238      238              
  Lines       24668    24668              
==========================================
+ Hits        20637    20640       +3     
+ Misses       4031     4028       -3     

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[bnjbvr]

Oh I see the PR is marked as draft, so I'll do another round of review when ready.

Review was one week ago, and there's been many pushes since then