Add a public signer/verifier interface for some common scenarios.

[Gh0u1L5]

Current transport/middleware design works well with native Go HTTP libraries. However, if I want to use some modern HTTP libraries like gin-gonic or resty, the user experience becomes a disaster. I have to clone *http.Request over and over and do a lot of dirty magic to prevent the middleware from corrupting my *gin.Context.

Thus I propose this Verifier/Signer interface, which gives more control for us to plug-in them to other HTTP frameworks.

Moreover, if I'm maintaining tens of thousands of crypto keys, it's impossible to pass them one by one when initializing a verifier. Thus I propose a VerifyingKeyResolver interface, so that the verifier can search a database by itself.