Add support for style=deepObject with query parameters in the Rust generator

[dmeijboom]

CC: @frol @paladinzh @farcaller @richardwhiuk

Add support for query parameters that use style=deepObject. ~It only works for objects so arrays are still not supported.~

Edit: this is no longer the case, arrays do work now :)

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[dmeijboom]

Tested with our setup and works as expected.

[wing328]

@dmeijboom thanks for the PR.

Tests passed via https://app.travis-ci.com/github/OpenAPITools/openapi-generator/builds/258552656

Let's give it a try.