Update signer's key_id getter to return KeyId instance instead of link

[wulfraem]

Description of change

Updates Signer trait's key_id getter to return Self::KeyId instance instead of a reference. This makes the getter more flexible, especially in cases, where the key id instance is not owned by the signer, and a reference would have to be created "on the fly".

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How the change has been tested

Compiled locally. Ran tests in identity.rs, that consumes the updated Signer trait.

Change checklist

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have checked that new and existing unit tests pass locally with my changes