feat(sdk-crypto): Add Identity based room key sharing strategy

[BillCarsonFr]

Draft because based on #3605
Fixes #3563

This PR just introduce a new way to distribute room keys based on identities, by only sharing with devices signed by their owner.
This is a first PR to introduce this mode, but it will be really usable once we introduce some support for identity TOFU.

• Public API changes documented in changelogs (optional)

Signed-off-by:

[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.

Project coverage is 83.87%. Comparing base (ca6537b) to head (9294618).
Report is 138 commits behind head on main.

Files	Patch %	Lines
...c/session_manager/group_sessions/share_strategy.rs	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3607      +/-   ##
==========================================
+ Coverage   83.86%   83.87%   +0.01%     
==========================================
  Files         254      254              
  Lines       25892    25909      +17     
==========================================
+ Hits        21714    21731      +17     
  Misses       4178     4178              

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[poljar]

Much better PR, thanks for taking the time to make this reviewable. I left some nits.

[poljar]

This method exists for Device

matrix-rust-sdk/crates/matrix-sdk-crypto/src/identities/device.rs

Lines 274 to 286 in 3be84a5

	/// Is this device cross signed by its owner?
	pub fn is_cross_signed_by_owner(&self) -> bool {
	self.device_owner_identity.as_ref().is_some_and(|device_identity| match device_identity {
	// If it's one of our own devices, just check that
	// we signed the device.
	ReadOnlyUserIdentities::Own(identity) => identity.is_device_signed(&self.inner).is_ok(),
	// If it's a device from someone else, check
	// if the other user has signed this device.
	ReadOnlyUserIdentities::Other(device_identity) => {
	device_identity.is_device_signed(&self.inner).is_ok()
	}
	})
	}

Please just move the body of that function over here instead of duplicating it.

[BillCarsonFr]

done

[poljar]

Let's return in each branch of that match a RecipientDevices struct, makes it a bit clearer that we're not really mutating multiple times the above vectors and might get rid of a couple of allocations.

Suggested change

	denied_devices_with_code
	RecipientDevices {
	denied_devices_with_code: user_devices
	.into_values()
	.map(|d| (d, WithheldCode::Unauthorised)),
	..Default::default()
	}

(You won't be able to apply that suggestion because I can't modify multiple lines and you need further modifications in the rest of the function).

[poljar]

Why the as_str() on both sides? This should be the same type, no?

[BillCarsonFr]

updated

[poljar]

Could you please preserve the comments from the other body of this function?

[BillCarsonFr]

done