Added support for sendContext

[alexjoybc]

Description

This PR includes the following proposed change:

• Support for MassTransit SendContext

This is more a start for a discussion, I have fork your lib and put it in one of my project.

I was something similar to sending via interface

public interface SubmitOrder
{
    string OrderId { get; }
    DateTime OrderDate { get; }
    decimal OrderAmount { get; }
}

public async Task SendOrder(ISendEndpoint endpoint)
{
    await endpoint.Send<SubmitOrder>(new
    {
        OrderId = "27",
        OrderDate = DateTime.UtcNow,
        OrderAmount = 123.45m
    });
}

and noticed that the uber-trace-id was missing in the message context:

"headers": {
    "uber-trace-id": ""
  },

I did a quick modification to your code to accommodate this, as you can see there's some duplication as I went and copy paste most of your code (really handy btw).

Type of change

• New feature

How Has This Been Tested?

I have tested this in a different project. no unit testing

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[yesmarket]

Hi alexjoybc,
Thanks for the feedback. I've added a couple comments to PR to fix duplication (which you mentioned).
In regards to the uber-trace-id on the message headers? You mentioned it's missing, but I'm not sure what this should be...

[alexjoybc]

Just seeing this now... I will have a look next week thanks for the feedback