Allow overriding the signing algoritm used for JWT client assertion

[janwijbrand]

Is your feature request related to a problem? Please describe.

I'd like to have control over the signing algoritms used for the JWT client asserstion authentication variants. The current API doesn't allow to override the default RS256, even thought the lower-level signing helper functions are parameterized.

Describe the solution you'd like

When setting up an PrivateKeyJWT() instance to register as client auth method, I'd like to pass in the signing algoritm I'd like to use.

I will reference a PR against maintain-0.15 branch for this feature request.

[lepture]

merged