[Rust Server] Support Bearer based Authentication

Created by: richardwhiuk

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.
  • @frol, @farcaller, @bjgill

Description of the PR

Add support for Bearer based authentication, in the client code generated by Rust Server. This is in addition to the API Key, and Basic authentication already supported.

This includes support for OAuth2 based authentication.

petstore-with-fake-endpoints-models-for-testing.yaml already covers this, so the generated code has been updated.

This was developed by myself and others at @Metaswitch