JCLOUDS-633: Support passing bearer token directly for OAuth2

https://issues.apache.org/jira/browse/JCLOUDS-633
Work in progress, I will be adding some tests soon but wanted to expose this for review.
[minor] Add a property to `CredentialType` and store this there? I.e.
```
enum CredentialType {
  BEARER_TOKEN_CREDENTIALS("bearerTokenCredentials"), ...;

  private final String stringRepresentation;

  private CredentialType(String stringRepresentation) {
    this.strin...;
  }
```
Probably not worth it unless the conversion will be called frequently, though.