Allow passing claims_options in StarletteOAuth2App

[Galaxy102]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update
• Refactor
• Other, please describe:

Does this PR introduce a breaking change?

• Yes
• No

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This pull requests allows to put custom claims_options into the StarletteOAuth2App. This is needed for example in a multihomed Keycloak setup. Here, the users accesses Keycloak from a public URL (e.g. https://keycloak.localhost with a Reverse Proxy in front) which might not be reachable from the application (e.g. in a Docker Container). In that case, the program using authlib would access Keycloak directly using a private URL (e.g. http://keycloak:8080).
Here, the JWT iss would mismatch when retrieving the user_info, but the developer knows that it has two possible values: The public and the private URL. To suppress the validation error, the base function AsyncOpenIDMixin.parse_id_token allows setting custom claims_options, but in StarletteOAuth2App, that was impossible to do.

[lepture]

Could you also add it for Django and Flask? I would like to keep them the same API.

[Galaxy102]

Done

[lepture]

Thanks