Add support to provide a custom claim name as authorization claim field

[ldclakmal]

User may need to provide the claim name which is used for authorizations in JWT. By default, we use scopes claim for this [1]. But that can be changed, or this can be dynamic.
Ex. Okta uses scp as the claim.

[1] https://github.com/ballerina-platform/module-ballerina-jwt/blob/v1.0.3/jwt-ballerina/src/jwt/inbound_jwt_auth_provider.bal#L119

[ldclakmal]

Authorization logic is currently limited on the scope attribute in every authentication mechanism (Basic Auth, JWT, OAuth2, LDAP). We need to improve this current ABAC approach providing the flexibility to user to build up own logic of authorization without limiting to the scope attribute we have now. But, from an implementation perspective, we must think carefully about how we can make it generic against all the authentication providers we support now, and we are yet to support. Also, apart from "static values" of the given attributes, we may have to consider how we can handle "dynamic values" of the given attributes, which may be used for authorization decisions.