Improve introspection cache

[sprudel]

Main goals of this PR:

• allow to use introspection cache in axum
• improve caching behavior by using a moka which is optimized for such scenarios
• introduce a default TTL in minutes as the access token expiry is not desired from a security perspective

To achieve this I had to remove the Role generic type parameter introduced in #550 , as this was incompatible with the type erasure for the IntrospectionCache trait types.
But there are simple ways to achieve the same convenience on library user side.

[buehler]

Very nice, however: since this will result in a breaking change, I need some kind of description, how users of #550 can work around the removed role feature.

[sprudel]

@buehler where would you prefer to have such documentation?

it is basically a standard extension trait

enum Role {
  Admin,
  Client
}

trait MyExtIntrospectedUser {
    fn role(&self, role: Role) -> Option<..>;
}

impl MyExtIntrospectedUser for IntrospectedUser {
   fn role(&self, role: Role) -> Option<..> {
     // convenience impl here

  }
}

[buehler]

It would be nice to have this in the docs of ZitadelExtraTokenFields struct. There were you removed the role generic. As such, I can also include them as the migration path for the breaking change.

[sprudel]

@buehler I added the example both for the IntrospectedUser and ZitadelExtraTokenFields struct