[JAVA] Feature add support for two-legged OAuth2

Description

Referencing to Google's documentation; in the case of authenticating server-to-server applications with two-legged OAuth2 is possible. This requires sending grant_type and assertion parameters to token_url.

Swagger declaration file content

I have already sent a PR to APIs-guru for adding token_url to their yaml definitions. See https://github.com/APIs-guru/openapi-directory/pull/116

Suggest a Fix

An additional OAuth flow can be added;

public void setFlow(OAuthFlow flow) {
        switch(flow) {
        // ...
        case application:
            tokenRequestBuilder.setGrantType(GrantType.CLIENT_CREDENTIALS);
            break;
        case twoLeggedAuth:
            tokenRequestBuilder.setGrantType(GrantType.JWT_BEARER);
            tokenRequestBuilder.setAssertion("assertion");
            break;
        default:
            break;
        }            
    }

assertion can be calculated as in this apache oltu example.

What do you think? If you want, I can give a try and send a PR.

Start by locating the Java OAuth flow handling around setFlow(OAuthFlow) and the tokenRequestBuilder. Review how token_url is represented and how GrantType values are sent, then compare the proposed application and twoLeggedAuth flows with the linked OAuth2 references. Done means server-to-server authentication can send the required grant_type and assertion parameters.