Add support for generating code for Spring RestClient

[jamezrin]

Missed an option to export requests using Spring Framework's RestClient, I haven't tested it in HTTPToolkit yet (I have yet to look what changes would be necessary) but the tests look good, the code is accurate.

I thought about maybe making the output runnable (within a method, imports, etc) but maybe it's a bit overkill?

[pimterry]

Thanks! This looks fantastic 👍

I don't think there's any need to make it fully runnable standalone, we don't do that for other Java snippets, and for most use cases I think this is perfect as-is.

I'll merge, release & pull into HTTP Toolkit now.