Transformations

https://github.com/microsoft/reverse-proxy/issues/60
https://github.com/microsoft/reverse-proxy/issues/21

Request

Request Parameters Transform - Applied before building the HttpRequestMessage

RequestParamtersTransformContext
- HttpContext
- Method
- Version? Or should that be controled by the destination?
- PathBase
- Path
- Query
- Exclude scheme and host/authority, those are set by the destination

RequestParametersTransform.Transform(UrlTransformContext context) - abstract/custom/base

PathStringTransformation:RequestParamtersTransformContext
- Value PathString (or path template that supports route values?)
- Prepend/Append/Set/RemoveStart/RemoveEnd Enum
- TransformPathBase/Path bool

PathTemplateTransformation:RequestParamtersTransformContext
- Value string route template (supports route values?)
- Replaces whole path
- TransformPathBase/Path bool (?)

QueryStringTransformation:RequestParamtersTransformContext - Use QueryStringHelpers.Append
- Value string
- Prepend/Append/Set Enum
- Transform(UrlTransformationContext)

Headers
- We don't want to change HttpRequest.Headers, nor do we want to deal with the split issue of HttpRequestMessage/Content.Headers.
- Idealy we'd transform headers as we copied from one object model to the other (if we are copying headers, that's a seperate flag).
  - foreach header in Request.Headers
    - Look up any transformations for this header, run them.
    - Set result on HttpRequestMessage/Content
    - Record header as transformed (hashset)
  - foreach transformation in transformations
    - If Key was not already transformed, run transformations

HeaderTransform
- Key - header name string
- Transform(HttpContext, existingValues, out newValues)

HeaderValueTransform : HeaderTransform
- Values StringValues
- Enum Append, AppendCSV, Set, etc...

Trailers - Same as headers, different collection

// Do we need this? Should this be done as HttpClient middleware instead?
// Things that can't be transformed in other ways: Properties, Body
RequestTransformation.Transform(HttpRequestMessage)

BodyTransform - MUST be custom middleware, out of scope

Response

// Do we need this? Should this be done as HttpClient or AspNetCore middleware instead?
// I would be nice to apply transforms before the response body started. HttpClient middleware can do that,
// but it's harder for AspNetCore middleware because you have to intercept the response body.
ResponseTransformation.Transform(HttpResponse) - Not HttpResponseMessage(?)

HeadersTransform - Same as request, applied during the copy loop
- Conditionally applied?

Trailers - Same as headers, different collection
- Conditionally applied?

BodyTransform - MUST be custom middleware, out of scope

# Initial feature
- Three collections
  - Request Parameters transforms
  - Request header transforms
  - Request trailers transforms (not currently supported)
  - Response header transforms
  - Response trailers transforms

# Nginx comparisons
- NGinx has a path regex based rewrite option: https://docs.nginx.com/nginx/admin-guide/web-server/web-server/#rewrite. This isn't part of pxoy request transformation, it's generic middleware.
- proxy_pass url/backend - if the backend defines a path base then it replaces the route matched path. Includes support for variaibles, which replace the original path.
- proxy_http_version - defaults to 1.0
- proxy_method (can contain variables)
- proxy_pass_request_headers - enables/disables forwarding all request headers
- proxy_set_header - set request header. Remove by setting empty. append/prepend using variables. sets Host to the destination host by default.
- proxy_hide_header - strip response header value. Has several defined by default.  proxy_pass_header allows defaults to be sent
- add_header/trailer - add response header (not proxy specific) - Only for specific status codes unless marked as 'always'
- proxy_cookie_domain/path - response Set-Cookie domain/path transformations
- proxy_redirect - response Location header prefix substitution