Add X-Forwarded-For header name

[DenisBalan]

tldr; add header name X-Forwarded-For as constant in HeaderNames static class.

more info https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-For

[davidfowl]

Why? We have middleware that already translates this into the appropriate values.

[DenisBalan]

Why? We have middleware that already translates this into the appropriate values.

This is useful when creating own http calls to other services, to not have magic strings, but reference to one source of truth ie HeaderNames

[benaadams]

Should be in PublicAPI.Unshipped.txt not Shipped

[ghost]

Thank you for submitting this for API review. This will be reviewed by @dotnet/aspnet-api-review at the next meeting of the ASP.NET Core API Review group. Please ensure you take a look at the API review process documentation and ensure that:

• The PR contains changes to the reference-assembly that describe the API change. Or, you have included a snippet of reference-assembly-style code that illustrates the API change.
• The PR describes the impact to users, both positive (useful new APIs) and negative (breaking changes).
• Someone is assigned to "champion" this change in the meeting, and they understand the impact and design of the change.

[Tratcher]

@dotnet/aspnet-api-review, I flagged this PR because I want to discuss and document what our policy is for adding constants to a list like this.

[Tratcher]

@DenisBalan we reviewed this and decided to add a written policy for what should and should not be added to this list. See https://github.com/dotnet/aspnetcore/pull/32394/files. Based on that policy I'm going to close this PR. Thanks for bringing this to our attention.