[cpp rest-sdk] Improve method signatures to use const when the value won't change

[whoan]

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Description of the PR

Improve method signatures to use const when the value won't change.

Ping @ravinikam @stkrwork @fvarose @etherealjoy @MartinDelille

[etherealjoy]

What about the const reference for setter arguments?

https://github.com/OpenAPITools/openapi-generator/pull/1295/files#diff-dcd1e3f1c138323826806ca0c2cb3465R56

Looks much better with this change, but you could perhaps consider this point too.

[etherealjoy]

I think it is a little complicated with Models, being passed a a shared_ptr.
You could fix it later if you wish.
Some work has been done for other clients and servers but I think it is going to be a breaking change.
So you could do it in the next step.

[wing328]

@whoan thanks for the PR, which is included in the v3.3.2 release: https://twitter.com/oas_generator/status/1057649626101112832