Custom request/response modifications via code transforms

[samsp-msft]

Ability to customize headers (beyond x-forward or affinity) from code extensibility for both request to back end, and response to the the client.

[Tratcher]

See feedback from #225 (comment) about config formats and how that may affect extensibility.

[AJAnderhub]

I would second the need for custom header modifications. Prepending to response headers would the use case for my scenario.

[robbieknuth]

+1, I have a need to modify headers in an ad-hoc fashion. Concrete scenario is ripping an auth token from a cookie and setting that as the Authorization header. This would eliminate my need for CORS entirely and simplify my api server to be jwt only, resulting in all client-ish things on the client server.

[Tratcher]

Requirements summary:

• Someone can write code to modify request and response parameters/headers.
• That code can be associated with specific routes via config, including parameters.
• Config validation can validate parameters for transforms.
• Transforms compose so you don't have to write one uber transform.
• Transformations need to allow async code.

Example scenarios:

• Swap auth format: Documentation for swapping authentication types #416
• Rename a header: Header transform to rename a header #327
• Rewriting redirects: 301 Redirection #478 How to get redirects right ? #466