Support for `X-Opaque-Id`

[philkra]

Feature request description:

• the feature is available as an opt-in, meaning that a dev needs to explicitly set the value of choice prior to executing the request from the client to elasticsearch. no value shall be generate automatically.
• after every request the populated header value needs to be removed, consequential requests must no longer contain the header if not explicitly set.
• provide a convenience setter method to set the header, e.g. set_opaque_id('xyz0123')
• usage example in the client's documentation
• setting the x-opaque-id header should be possible for every request/endpoint, since the ID is used in the slow logs as well as for bubbling up deprecation warnings.

Constraint: Elasticsearch >= 7

Relates to per-request custom HTTP header setting from #186