Add OAuth2 support

[jbonofre]

The purpose here is to add a new JAAS LoginModule allowing authentication based on JWT.

We can use Nimbus JWT dependency to deal with that.

The idea is:

1. Introduce a new OAuth2LoginModule to be used by the JaasAuthenticationPlugin, similar to the existing PropertiesLoginModule or LDAPLoginModule.
2. Validates tokens locally, to avoid systematic call to the IdP on every connection.
3. The OAuth2 configuration (issuer, claim, etc) is defined in the conf/login.config as other LoginModules.
4. The user pass the token as "password"