Add Azp claim interface

[mjudeikis]

This adds azp (authorized party) claim to the Claims.

We ran into the issue where using OpenID this field comes up very often. It's not part of JWT spec, but it is part of OpenID (https://openid.net/specs/openid-connect-core-1_0.html#IDToken) Especially in Azure AAD and Auth0. When doing nested OIDC providers. And we are not fully able to validate claims as depending on which authorization method you use they are mixing aud and azp fields 😿

This is very similar to #352

[mjudeikis]

cc: @oxisto

[oxisto]

Unfortunately, we can not accept new claims to the Claims interface, because this is not backwards compatible. I would suggest using a custom claim that embeds one of the standard claims, as can be seen in this example:

jwt/example_test.go

Lines 37 to 40 in 6bcdd9d

	type MyCustomClaims struct {
	Foo string `json:"foo"`
	jwt.RegisteredClaims
	}

[mfridman]

I think this class of issues is one of the most common. We should probably update the README.md and the docs site (maybe an FAQ) to explicitly call it out with a copy/paste example.