Allow creation of GivenKeys with a required algorithm

[sean-rn]

To comply with RFC 8725 Section 3.1, we allow specifying an alg for GivenKeys. It behaves equivalently to the 'alg' parameter parsed from JWKS JSON.
Resolves #69

• I implemented it as a new function for backwards compatibility. The same sort of overload could be made for all the other constructors if desired.
• I considered if we needed a way to specify use as well as alg but I think not. If a key's use doesn't apply, simply don't add that key.
• I added JWKS.KeyAlg() for symmetry and convenience.
• Unit tests cover both the happy path and the rejection of a mis-matching algorithm.

Please feel free to rename either function to better fit your conventions.

[MicahParks]

Thank you! I will likely review this tomorrow.

[MicahParks]

Thank you for noticing the PR and gap in implementation, @sean-rn! I'll merge this now and do a release later after a few edits. The next release may be sometime today.