[Rust][client] Partly fix bug #16974 - add missing imports

[j-szulc]

Note: this PR is not complete as there are issues I cannot do on my own (see below), hence draft PR.

It partly fixes the related issue #16974.
This PR simply adds a line to the top of *_api.rs importing the models used as arguments of the endpoint functions.

The issue is that:

1. If a model is used multiple times, it's being imported multiple times, which causes an error,
2. There are warnings that some imports are unused (maybe goes away after fixing 1. above)

As Mustache is a logic-less template engine, to filter out a unique list of models requires delving into the enormous Java codebase of this project, which I can't do.

Mentioning technical committee per PR checklist: @jacob-pro

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[DDtKey]

I think the issue not imports itself, clients use clarified path to models, i.e models::MyStruct. Usually there is no need for explicit imports here.

This change creates a lot of extra imports, for example affected apis by this PR - worked fine even before, but check/clippy will highlight that there are extra clarified paths.
Moreover imports not unique, eg:

openapi-generator/samples/client/petstore/rust/reqwest/petstore-async-middleware/src/apis/pet_api.rs

Lines 15 to 16 in 65daa15

	use crate::models::Pet;
	use crate::models::Pet;

I think the issue is this condition: {{#isBodyParam}}models::{{/isBodyParam}} (e.g https://github.com/OpenAPITools/openapi-generator/blob/a8efb8eea8a8136311db2fa4dfea6df39d1c3d4a/modules/openapi-generator/src/main/resources/rust/reqwest/api.mustache#L93C483-L93C523)

So we have 2 questions:

1. why it's not true for these models?
2. for hyper we don't use this condition and it seems to work

UPD: I've submitted this PR: #17968