Refactor Provider to specify custom URIs for API mount points

[dmitrizagidulin]

Currently, the various OIDC-related API endpoints are hardcoded in the Provider's constructor:

    data['authorization_endpoint'] = `${issuer}/authorize`
    data['token_endpoint'] = `${issuer}/token`
    data['userinfo_endpoint'] = `${issuer}/userinfo`
    data['jwks_uri'] = `${issuer}/jwks`
    data['registration_endpoint'] = `${issuer}/register`
    data['check_session_iframe'] = `${issuer}/session`
    data['end_session_endpoint'] = `${issuer}/logout`

(This also affects how they're advertised in .well-known/openid-configuration).

It would be preferable to allow a consumer app to specify which URI path to mount these endpoints on. Sort of like:

let provider = new Provider({
  issuer: 'https://example.com',
  mount: '/oidc/'
})
// -> which would lead to:
// https://example.com/oidc/authorize
// https://example.com/oidc/token
// https://example.com/oidc/userinfo
// etc

Note: This is only a refactoring of the provider's discover() functionality, not anything to do with OP express routes.