Allow for user provided ctx during parse

The purpose of this release is to add a new method, .KeyfuncCtx.

This new method accepts a context.Context, then returns a jwt.Keyfunc. This user provided context.Context is used during JWK lookup in the github.com/MicahParks/jwkset package when parsing JWTs. Passing a request scoped context allows the JWT parsing and JWK retrieval to cancel according to the given context.Context behavior instead of the default context.Context, which was provided at keyfunc.Keyfunc initialization.

In practice, this is used to prevent situations where many JWTs with kid not in a remote JWK Set are attempting to be parsed over a long period of time.

Relevant issues:

• MicahParks/jwkset#26

Relevant pull requests:

• #118
• #119
• #120
• #121