OZ-528: Add Oauth2 authentication support to openmrs-fhir component.

[corneliouzbett]

This PR introduces Oauth2 authentication support to the openmrs-fhir component.
The OpenmrsFhirConfiguration class has been updated to include a condition to check if Oauth is enabled and accordingly set the Authorization header for the FHIR client.

[ibacher]

Nice work! A few high-level comments:

• This will fetch a new OAuth2 token for every request. Since OAuth2 tokens have an expiry, it would be better if we could cache the token for a period of time (because without that, this effectively means that each FHIR API request requires two HTTP requests).
• The OauthProcessor checks that the returned token_type in the response is, in fact, a bearer token. While, for the basic Ozone setup, this should work, it may be best to double-check that.
• It would be good in the OpenmrsFhirOauth2 component to actually check that the supplied values are present if OAuth is enabled (or at least when calling fetchAuthToken(), since otherwise, this is likely to generate very opaque error messages.

[Ruhanga]

Do we have to reinvent the wheel, why not simply reuse the existing OauthProcessor to handle the auth processing (at-least everything involving the Bearer Token)?

[corneliouzbett]

Do we have to reinvent the wheel, why not simply reuse the existing OauthProcessor to handle the auth processing (at-least everything involving the Bearer Token)?

The OauthProcessor was designed to be used within routes. It's structured in such a way that it fetches the token as part of a route. Leveraging this means invoking the producer template/accessing camelContext within the interceptor and then extracting the token from the message body. This could potentially introduce unnecessary complexity.