Add support to configure api-key for avro schema registry

[vineethpp]

I came across a scenario, in which the avro schema registry hosted requires api-key to be provided by client in requests and if any client is sending request without api-key the requests fails. The final request to avro schema registry is expecting to include api-key to be provided as query param in final url or in header key x-api-key, for example which looks like - https://schemaregistry.myorg.com/subjects?key=123456789101112131415

It would be good to add ability to configure and send the key as a query param in final url of avro schema registry calls and the config name can be schema_registry_apikey or schema_registry_api_key the value configured will be send in the url query param or in header key x-api-key for all requests to schema registry.