Allow overriding the OAuthToken class.

Hello,

To integrate this bundle inside OroCRM, I need to have my own Token that will extend OAuthToken and implement `Oro\Bundle\SecurityBundle\Authentication\Token\OrganizationContextTokenInterface`.

To do that, I need to be able to change every creation of `OAuthToken` class by my own.
How would you do that ? Create a Factory class ? WDYT ?
should be private