refactor: use struct types for claim related types

BREAKING change.
The following types are changed from interface to struct type:

- AccessTokenClaims
- IDTokenClaims
- IntrospectionResponse
- UserInfo and related types.

The following methods of OPStorage now take a pointer to a struct type,
instead of an interface:

- SetUserinfoFromScopes
- SetUserinfoFromToken
- SetIntrospectionFromToken

The following functions are now generic, so that type-safe extension
of Claims is now possible:

- op.VerifyIDTokenHint
- op.VerifyAccessToken
- rp.VerifyTokens
- rp.VerifyIDToken

README.md

@@ -98,9 +98,7 @@ Versions that also build are marked with :warning:.
 
 | Version | Supported          |
 |---------|--------------------|
-| <1.16   | :x:                |
-| 1.16    | :warning:          |
-| 1.17    | :warning:          |
+| <1.18   | :x:                |
 | 1.18    | :warning:          |
 | 1.19    | :white_check_mark: |
 | 1.20    | :white_check_mark: |