Secret storage support in the main crate

[poljar]

This PR utilizes the cryptographic primitives for secret storage support introduced in #2591, and adds a convenient and easy to use API on top of it. As such, this PR depends on #2591.

We're now able to import the private cross-signing keys and verify our own device by entering the secret storage key or passphrase.

The PR should be reviewed commit by commit. An example, in case somebody wants to play around with this as part of the review, is provided as well.

• Public API changes documented in changelogs (optional)

[Hywan]

I like this PR. The code is clean, there is plenty of doc', really good job.

I believe that it may be useful to be able to remove a secret from the store though. What do you think?

[codecov]

Codecov Report

Attention: 21 lines in your changes are missing coverage. Please review.

Comparison is base (61eb9ce) 81.35% compared to head (38e10db) 81.36%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #2621    +/-   ##
========================================
  Coverage   81.35%   81.36%            
========================================
  Files         204      207     +3     
  Lines       21051    21172   +121     
========================================
+ Hits        17126    17226   +100     
- Misses       3925     3946    +21     

Files	Coverage Δ
crates/matrix-sdk/src/client/mod.rs	87.91% <ø> (ø)
crates/matrix-sdk/src/encryption/mod.rs	71.42% <100.00%> (+0.21%)	⬆️
...es/matrix-sdk/src/encryption/secret_storage/mod.rs	100.00% <100.00%> (ø)
...atrix-sdk/src/encryption/secret_storage/futures.rs	73.68% <73.68%> (ø)
...-sdk/src/encryption/secret_storage/secret_store.rs	78.37% <78.37%> (ø)

... and 4 files with indirect coverage changes

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[poljar]

I rebased this now and changed the base branch to #2758.

[dkasak]

Discussed on a call with poljar out-of-band. Left some minor comments and suggestions but this otherwise looks really good. There is one important suggestion, which is a zeroization fix.

After the suggestions are resolved, LGTM.

[dkasak]

Some suggestions.

EDIT: It seems that Github is bugged and has eaten my suggestions alive :(

[poljar]

Sorry this is taking too long. If you still remember the suggestions feel free to provide them as a patch.