Add support for OAuth 2.0 to Flowable's REST APIs

**Is your feature request related to a problem?**
Contemporary application's use OpenID Connect for Authentication and OAuth 2.0 (scopes and claims) for Authorization.

**Describe the solution you'd like**
Add support for OAuth 2.0 to Flowable's REST APIs.

See:
- [Support Basic Auth and Spring OAuth 2.0](https://stackoverflow.com/questions/23526644/spring-security-with-oauth2-or-http-basic-authentication-for-the-same-resource)
- [Securing your REST API using Keycloak and Spring OAuth2](https://medium.com/@bcarunmail/securing-rest-api-using-keycloak-and-spring-oauth2-6ddf3a1efcc2)

**Describe alternatives you've considered**
Currently, Flowable only supports Basic Auth.

See: https://github.com/flowable/flowable-engine/issues/660#issue-274448784

**Additional context**
See: [Keycloak, Flowable and OpenLDAP](https://robferguson.org/blog/2020/01/03/keycloak-flowable-and-openldap/)