Add support for custom JWS algorithms

[frederikrothenberger]

This PR introduces a feature custom_alg to identity_jose (disabled by default) that allows it to process JWS with custom alg values.

Switching on custom_alg makes quite a few changes to JwsAlgorithm:

• The type is no longer Copy
• name() takes only a reference and returns a String rather than &'static str
• The constant ALL is removed as it is no longer possible to enumerate all variants

Fixes issue #1406.

Type of change

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

• [ ] Bug fix (a non-breaking change which fixes an issue)
• [x] 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

Added a new integration test to identity_jose that exercises the functionality.

Note: requires CI changes to be run with cargo test --test custom_alg --features="custom_alg"

Change checklist

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

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

[frederikrothenberger]

@itsyaasir, @eike-hass: The change using a feature flag seems to be quite reasonable. What do you think?

[itsyaasir]

Thanks for this @frederikrothenberger