[Rust][Client] Unify sync/async client structure

Created by: bcourtine

Unify structure:

• Remove xxxApiClient structures and traits.
• Configuration is now passed as function argument (instead of being part of the client struct).

This evolution makes much easier the sync/async switch since structure and signatures are now much closer.

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project beforehand.
• Run the shell script ./bin/generate-samples.shto update all Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/config/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

cc @frol (2017/07) @farcaller (2017/08) @richardwhiuk (2019/07) @paladinzh (2020/05) @wing328

Also cc @HenningHolmDE @seunlanlege since you are currently the main rust generator users.

added Breaking change (without fallback) Client: Rust labels

changed milestone to %5.0.0

closed