Add generic getClaim() method in ClaimAccessor

[jgrandja]

We should add a convenience (default) method in ClaimAccessor, as follows:

default <T> T getClaim(String claim) {
    return (T) this.getClaims().get(claim);
}

Related to comment

[ThomasVitale]

Hi @jgrandja, if needed I would be available to work on this issue

[jgrandja]

@ThomasVitale That would be great! It should be pretty straight forward - similar to what we already have in OAuth2AuthorizationRequest.getAttribute() (in master). Let me know if you have any questions. Thanks.

[ThomasVitale]

Hi @jgrandja, I created a PR for the issue, what do you think?