Add C# HttpClient target client

[Atvaark]

Adds a second C# target client, one that doesn't require an additional NuGet package to work.
This will generate a C# snippet which can be pasted into the body of an existing (async) method.
#141

Differences between this and PR #142:

• this also generates working code for multipart/form-data requests
• this supports custom HTTP methods
• this enables automatic gzip, deflate (but no br) decompression if a compatible accept header is found
• this generates code similar to the restsharp target client: no method declaration boilerplate is generated and the user has to add the required using statements on their own

[PavelStefanov]

Hey guys! This PR looks nice and really useful, can we merge that? If you need any help feel free to ask me, I'd be happy to help.
I'd suggest make HttpClient the default target

[Atvaark]

Would anyone mind taking a look at this? The CI and tests work again and there aren't any conflicts.
@reynolek @darrenjennings