add datadog-origin header

[ojproductions]

What does this PR do?

Adds the datadog-origin header to context if its present in given event headers or extension context

As noted in Issue #189, in the case of synthetics, if the origin header isnt present in the extractTextMap function in dd-trace-go, an ErrSpanContextNotFound error occurs due to the missing origin:

By including datadog origin header, we fix this:

Motivation

Issue #189
APMSVLS-1

Testing Guidelines

I created some unit tests and updated existing ones. Also ran lamba functions to see if expected header was present

Additional Notes

Types of changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Checklist

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog

[joeyzhao2018]

Also ran lamba functions to see if expected header was present

nitpik for this: it would be nice to have some screenshots or links to view, validate and understand. In this specific case, I was wondering where the headers are showing in the UI? is it a tag of a span?

[joeyzhao2018]