feat(crypto): Customized event types

[poljar]

This patch adds customized event types, currently only for the m.room_key and m.secret.send to-device events.

This allows us to:
a) Deserialize the session_key field into a vodozemac type
b) Control when we zeroize secrets better

[codecov]

Codecov Report

Merging #767 (4a198d1) into main (f0190b4) will increase coverage by 0.02%.
The diff coverage is 68.77%.

❗ Current head 4a198d1 differs from pull request most recent head e6bde16. Consider uploading reports for the commit e6bde16 to get more accurate results

@@            Coverage Diff             @@
##             main     #767      +/-   ##
==========================================
+ Coverage   77.92%   77.95%   +0.02%     
==========================================
  Files          92       96       +4     
  Lines       13930    14088     +158     
==========================================
+ Hits        10855    10982     +127     
- Misses       3075     3106      +31     

Impacted Files	Coverage Δ
crates/matrix-sdk-crypto/src/store/mod.rs	53.73% <0.00%> (+0.39%)	⬆️
crates/matrix-sdk-crypto/src/types/mod.rs	81.53% <ø> (ø)
crates/matrix-sdk-crypto/src/machine.rs	80.02% <60.00%> (+3.24%)	⬆️
...es/matrix-sdk-crypto/src/types/events/to_device.rs	61.73% <61.73%> (ø)
crates/matrix-sdk-crypto/src/gossiping/machine.rs	80.40% <70.58%> (-0.06%)	⬇️
...tes/matrix-sdk-crypto/src/types/events/room_key.rs	74.28% <74.28%> (ø)
.../matrix-sdk-crypto/src/types/events/secret_send.rs	82.35% <82.35%> (ø)
.../matrix-sdk-crypto/src/verification/event_enums.rs	67.51% <89.47%> (-0.24%)	⬇️
crates/matrix-sdk-crypto/src/olm/account.rs	82.29% <100.00%> (-0.05%)	⬇️
...atrix-sdk-crypto/src/olm/group_sessions/inbound.rs	88.32% <100.00%> (-0.09%)	⬇️
... and 6 more

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[jplatte]

Why is this code working with mutable events in the first place? That strikes me as a little odd.

[poljar]

This is explained here:
https://github.com/matrix-org/matrix-rust-sdk/pull/767/files#diff-723b3a87aa508f41a037f9c4222c6c2e125abca051e3d60c43cb99e275ebcfe7R783=

// Set the secret name so other consumers of the event know
// what this event is about.

That event type doesn't really carry enough information to let users know what the secret is about. You only get to know this via previous state, from the time you made the request.

For convenience we set the secret name.

[jplatte]

Since this is not a collection type (i.e. an instance only contains one event), its name should be singular, right? We could use the same naming as Ruma (AnyToDeviceEvent).

[poljar]

I agree, but having two AnyToDeviceEvent types in the crate would confuse me to no end. Eventually we'll use only our customized one in the matrix-sdk-crypto crate and we'll rename it then.

So bear with me until we get to that point.

[poljar]

Rebased onto main to get CI going.

[Hywan]

Looks good to me, thanks for having taking time to do it.