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

[wulfraem]

Description of change

Updates Signer trait's implementation to return Self::KeyId instance instead of a reference in key_id getter to follow updated trait. See related PR here.

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", as would have been the case for our WASM implementation.

Current (as of the time opening the PR) references point to PR branch to let CI run its checks. This will be changed to a tag or to the main branch, depending on how soon we tag the changes in secret-storage.

Links to any relevant issues

• fixes figure something out for the fn key_id(&self) -> &String in the signer, should we: #1595

Type of change

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• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

Ran tests and examples locally.

Change checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes