Permit construction of a GivenKeys map from JSON

[sean-rn]

This simplifies the use case of supplementing keys loaded from a JWKS url with ones parsed from static JSON.

Pseudocode for how to use it:

staticKeys, err := keyfunc.NewGivenKeysFromJSON(staticJSON)
combinedKeys, err := keyfunc.Get(jwksURL, keyfunc.Options{GivenKeys: staticKeys})

[sean-rn]

Draft pending discussion on #73

[sean-rn]

I don't love the function name keyfunc.NewGivenKeysFromJSON. Maybe just keyfunc.GivenKeysFromJSON is better.

[MicahParks]

I don't love the function name keyfunc.NewGivenKeysFromJSON. Maybe just keyfunc.GivenKeysFromJSON is better.

Let's leave it as keyfunc.NewGivenKeysFromJSON. GivenKeys isn't a specifically defined type by keyfunc, but I think it still makes sense as a name.

You can mark this PR as ready if you feel the changes are complete. I think these changes look great!