sudo/sessionKeys: Implement sudo_sessionKeys_unstable_generate

[lexnv]

This PR implements the sudo_sessionKeys_unstable_generate method.

This is basically an equivalent for the rotate_keys API in the RPC-Spec-V2 with extended ability to provide a user-defined seed.

The RPC method calls into SessionKeys_generate_session_keys runtime function.
Then, the SessionKeys_generate_session_keys runtime function generates a series of keys,
inserts those keys into the keystore, and returns all the public keys concatenated.

Added a test to validate that keys are propagated to the keystone.

This PR does not expose the sudo_sessionKeys_unstable_generate to the RPC layer yet, since there might be changes wrt the naming convention.

Part of #1516.

@paritytech/subxt-team

[tadeohepperle]

Great work! Looks good to me :)

[bkchr]

@lexnv what is the status here?

[lexnv]

I believe I put this on hold for the new SessionKeys::generate_session_keys(owner: Vec<u8>, seed: Option<Vec<u8>>) method introduced in:

• frame-session: Introduce a proper proof of key ownership #1739

Since this PR is already spec-ed out and will add it as _unstable, we can easily modify it later.

If that sounds like a plan, I'll polish this up a bit and ask for some reviews soon 🙏

[bkchr]

Okay 👍 Yeah I will finish the mentioned pr soon.

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs.
Job name: cargo-clippy
Logs: https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/6881158