[swift5][client] add support for async await

[4brunu]

With the release of Swift 5.5, Swift now supports async await.

This PR adds support for async await in the generated code.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  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/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (5.3.0), 6.0.x
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

@jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @4brunu (2019/11)

[4brunu]

The CI issue is because our bitrise config is using an old version of Xcode, Xcode 11.7.x.
This feature requires Xcode 13, that was released yesterday.
I'm not sure if this is already available in bitrise.
Ideally we should upgrade to Xcode 13 if it's already available in bitrise, if it's not available yet, I suggest disable the CI job just for the new sample project of async await, and enable it once Xcode 13 is available on bitrise.
But I think we shouldn't hold this PR because of this CI issue, since it's a nice addition to the generator that is creating a lot of hype in the swift community.
I tested it on my machine with Xcode 13 and it works.

[wing328]

@4brunu nice PR 👍

What about skipping the test of new async await in the Bitwise CI for the time being and test it locally instead ?

[4brunu]

@wing328 looks good to me.
I have applied your suggestion and CI is now passing 👍