csharp-netcore: Add HttpClient support

[devhl-labs]

This is the beginning changes to move to HttpClient. It was working for my use case before I made the ApiException generic. At the moment it requires some post processing to replace ???? to ? and ??? to ?. This can be appended to the powershell script that begins the creation process in the first place. If this is good, then there is a lot of work to be done.

#6800
#5375

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[wing328]

@devhl-labs thanks for this. What about adding the library option and let users choose which HTTP libraries to use? That's something we've already done in a few other generators, e.g. java, ruby.

[devhl-labs]

There is no interface on HttpClient so it's hard for me to imagine how to do that gracefully. Plus HttpClient is very versatile. Even in the RestSharp library people are asking for it to use HttpClient, though it doesn't look like it will happen.

[devhl-labs]

Also I'm guessing the changes I'm made would lead toward a mass rewrite, scrapping almost everything. That's why I put this pr here before I do such a thing. I doubt I'd migrate all the supported features since I only use a small subset.

[wing328]

I saw the changes you made and we can easily support those via library option (which uses a new set of mustache templates) instead of using {{#isHttpLibrary}} .... {{/isHttpLibrary}} to handle the logic in all the existing template.

I'll file a PR to add the library option and we can merge this one into that one to continue the work.

[devhl-labs]

Oh like a set of mustache files for RestSharp and a whole other set for HttpClient? Yes that would be ideal as it wouldn't break for existing users.

[wing328]

Right. I'll try to file the PR in the afternoon and keep you posted.

[wing328]

And thanks for starting the work on this. Much appreciated.

[wing328]

I'll merge this into the branch csharp-netcore-httpclient to make it easier to collaborate on adding HttpClient support.

[wing328]

I've added the library option (httpclient) in the branch. Please have a look at #8331 and let me know if you've any question.