Add X-Elastic-Product header on all http responses

[jbaiera]

We should add a header to Elasticsearch responses that confirms the type of service operating on the other end of the connection. This will aid in validating client-server connections to ensure protocol compatibility between requestors and the server.

The header should return from all requests and its contents will likely be as follows:

X-Elastic-Product: Elasticsearch

[elasticmachine]

Pinging @elastic/es-core-features (Team:Core/Features)