Support for ControlMessage as an output type for HttpServerSourceStage and HttpClientSourceStage

[dagardner-nv]

Description

• Add optional message_type, task_type, task_payload arguments to HttpServerSourceStage & HttpClientSourceStage.
• Add new ConfigurableOutputSource base class for single output source stages which support both MessageMeta and ControlMessage as output types.
• When ControlMessage output is selected, HTTP request headers will be added to the message's metadata.
• Define new request handler callback method request_handler_fn_t which receives the boost asio tcp endpoint along with the request object.
• HttpEndpoint now optionally receives a request_handler_fn_t method, and for compatibility the older payload_parse_fn_t method.
• Add an optional include_headers argument to HttpEndpointInterfaceProxy::init, which when False (the default) uses the existing payload_parse_fn_t method, and request_handler_fn_t when True. This changes the function signature, but not the behavior when the include_headers argument is omitted, so technically not an API breaking change.
• Move/rename cm_task_t-->control_message_task_t from deserialize.hpp to be shared with other stages
• Add a new constructor for ControlMessage which receives a MessageMeta
• Don't use accept_status for live and ready endpoints, as these should always return 200 on success, while accept_status defaults to 201 CREATED which isn't appropriate for a health check.

Includes isort fix from #1842
Closes #1811

By Submitting this PR I confirm:

• I am familiar with the Contributing Guidelines.
• When the PR is ready for review, new or existing tests cover these changes.
• When the PR is ready for review, the documentation is up to date with these changes.

[dagardner-nv]

/ok to test