feat: add encrypt_targeted_message() [WPB-27214]

[SimonThormeyer]

This is further partial work for WPB-27214.

Remaining after this

• nonce handling on the receiver side
• decrypt_targeted_message()
• encrypt/decrypt tests

[fewerner]

Not much to add from my side. GJ so far!

[istankovic]

We still have CoreCryptoMessage in a few places:

crypto/src/mls/conversation/mutable/tnt/targeted.rs:70:    pub(super) const SIGN_LABEL_PERSISTED: &str = "CoreCryptoMessageTBS-Persisted-Targeted";
crypto/src/mls/conversation/mutable/tnt/targeted.rs:71:    pub(super) const SIGN_LABEL_TRANSIENT: &str = "CoreCryptoMessageTBS-Transient-Targeted";
crypto/src/mls/conversation/mutable/tnt/encrypt_targeted.rs:69:            .map_err(OpenMlsError::wrap("signing CoreCryptoMessageTBS"))?;
crypto/src/mls/conversation/mutable/tnt/encrypt_targeted.rs:73:            .map_err(Error::tls_serialize("CoreCryptoMessage: targeted"))

[istankovic]

Looking good! 🍷