External credential provider support for headers and query string parameters

[emgarten]

External credential providers should be able to contribute headers and query string parameters to http requests made by HttpSource to enable auth for additional protocols beyond basic auth.

Auth support in NuGet is currently limited to NTLM and username/password credentials using NetworkCredential.

https://github.com/NuGet/NuGet.Client/blob/3cfa0d51e338c32aa16ac05098cd7b74d595717c/src/NuGet.Core/NuGet.Protocol/HttpSource/HttpSourceAuthenticationHandler.cs#L70-L71

Examples

Azure uses SAS tokens which are added to the query string
https://docs.microsoft.com/en-us/azure/storage/common/storage-dotnet-shared-access-signature-part-1

AWS allows adding an authorization header to requests
https://docs.aws.amazon.com/AmazonS3/latest/dev/RESTAuthentication.html

[natemcmaster]

Just FYI - this would be immensely helpful for (ASP).NET Core's infrastructure.

cc @mmitche

[emgarten]

@nkolev92 how difficult would it be to add support for query params/headers now that credentials providers are using the plugin protocol?

[nkolev92]

@emgarten
The majority of the work with the new plugin has to do with the acquisition and reliability of external plugins.
We still only use the NetworkCredential model.

So we're not any closer from a network protocol side, but getting that info to NuGet, adding an extra capability to the plugin would be super easy.

[emgarten]

From what I recall changing the credential provider in the client protocol layer didn't look too bad. It just needs to allow storing more than a system NetworkCredential.

Good to know that adding a plugin capability for this would not be difficult, I thought that would be most of the work.