Support multiple content types

[afsalthaj]

I had errors of Unimplemented, Content with not exactly 1 option. and I thought I will quickly fix this.

The change was done as part of golemcloud/golem#1049

• CLI can do serde_json::from_str(a_yaml_file) and use the the REST API with content-type as json itself. So this is not for golem-cli to work with latest changes that support YAML, but rather make this client generation successful without compromising the details in the spec (refer the other PR). That is, in the PR, golem-service.yaml has details of API-definition import that says it supports both xml and json.

• With this new spec, the client generation failed with an error mentioned above. This PR is just enough reasonable changes to get over this problem. Currently it supports multiple content type for 1 single endpoint handling only yaml or json. If anyone needs more , they can add. Hopefully this scenario is not going to come often.

• The changes are tested and now works: the branch is in use in PR Yaml support for API definition golem#1049

TODO

• Cleanup

[afsalthaj]

Part of #1035