[csharp-netcore] Add null reference types and experimental http client library

[devhl-labs]

Adding NRT option and templates for experimental library.

The commits between the verified commits should be undone by 5b7c5da. Sorry for this. My fork got a bit messy because my last pr was from my master, and I had my template files in httpclient instead of httpclient-experimental folder.

@wing328 @Blackclaws

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[devhl-labs]

@wing328 Can i get some input here? I can move this back to the other branch if that helps. I've got some other changes to make but there's quite a bit here to see.

[wing328]

@devhl-labs can you please PM me via Slack to further discuss this? Thanks again for the PR to improve the C# generators.

[lucamazzanti]

Is this PR an obsolete alternative to #9235?

[Blackclaws]

Its a more involved rewrite of the csharp client and the null reference part was split out. We'll actually need to review that.