Support deprecated API endpoints in reqwest-based Rust clients

[hovinen]

Previously, if an API endpoint was marked deprecated, this fact was not reflected in the generated Rust clients using the reqwest library. We want to know exactly when our client code is using a deprecated endpoint, so marking the corresponding functions with the #[deprecatd] attribute would be very helpful. Uses of the endpoint would then be picked up by linters so that we can react.

This adds a line to the template which marks functions generated from deprecated endpoints with the #[deprecated] attribute.

This does not touch any of the other client generators for Rust or any other language, since they are out of scope for our needs.

Unfortunately, even after about a dozen attempts, I was unable to coax Mustache into not introducing an extra vertical whitespace when the #[deprecated] attribute is absent. This has no effect on functionality of the generated code, but is visually unappealing.

@frol @farcaller @richardwhiuk @paladinzh @jacob-pro @dsteeley

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[wing328]

thanks for the contribution, which has been merged

[hovinen]

thanks for the contribution, which has been merged

Thank you!

One question: When will there be a new release containing this change?

[wing328]

please refer to the milestone page: https://github.com/OpenAPITools/openapi-generator/milestones