[Keycloak-js] Expose processCallback, parseCallback and createPromise to custom adapters

[javialon26]

Possible solution to issue #10174.

I'm adding a new way to create adapters, which is a factory function (receive as parameters all the required internal KC methods like processCallback, parseCallback, createPromise, and KC instance) and must return a KeycloakAdapter object.

This is a preliminary PR, we need to work on tests, documentation, etc. but the idea is pretty clear.

[jonkoops]

Thanks for the PR, but we do not want to expose createPromise() as a public API. Keycloak JS should be refactored internally first to move towards standard Promise-based code.