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, since this is a test] Would
```
if(!System.getProperty(OAuthProperties.CREDENTIAL_TYPE, "")
    .equalsIgnoreCase(CredentialType.BEARER_TOKEN_CREDENTIALS.toString())) {
```
also work here? I'm assuming `""` will *not* be equal to BEARER_TOKEN_CREDENTIALS, so this would also be `false` if the property is not present.