Updated signing logic

[dtsiflit]

Description of change

The BindingKey jwk case now accepts SigningKeyProxy instead of a SecKey as an associated value.
This is a breaking change and to continue using keys please use .secKey(your_key) instead of just your_key when creating a BindingKey.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• 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 readme
• My changes generate no new warnings
• I have added unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes