DCR: Add ability to Add/View Http Headers

[daveta]

Currently, by-design, there isn't a way to get Http headers (both incoming from channel, or adding http headers outbound).

There have been a few cases (specifically around custom correlation id scenarios) to:

• Inspect Http Headers coming in, and be able to access in Middleware components
• Be able to add Http Headers when sending messages

For correlation/tracing, I believe this mostly goes away App Insights adopts an in-flight standard. However, current ETA is December, which may slide.

https://w3c.github.io/distributed-tracing/report-trace-context.html
microsoft/ApplicationInsights-dotnet-server#945

I'm filing this because I believe Stevenic mentioned this was requested before (not sure the scenario) and potential slippage in finalization of the standard.

[drub0y]

⚠️ HTTP is just one possible transport and I would highly recommend we be very careful about leaking anything specific into the Bot Builder APIs that mention or expose HTTP'isms directly. ⚠️

At most you'd maybe shove something into the TurnContext::Services collection at the adapter level that downstream middleware could look for and be tolerant to not finding in case, somewhere down the line, we support other transports.

[gabog]

Closing as part of R10 triage.