Implement standard message send operations

[jplane]

The SendMessage operation is stubbed out but not yet implemented.

We'd like a few things here:

• a standard HTTP send message implementation (should handle HTTP verbs, query strings, body payloads, etc.)
• a means for users to configure their own send message implementations prior to execution, probably using an ExecutionContext hook
  • probably need to add interface(s) to the abstractions assembly
  • this could include replacing the standard HTTP implementation with something better
• needs to work for both in-proc and durable task implementations