Add support for Awaitable and Awaitable<T>

[sailro]

Fixes #255

Checklist

• I have read the Contribution Guide ;
• There is an approved issue describing the change when contributing a new analyzer or suppressor ;
• I have added tests that prove my fix is effective or that my feature works ;
• I have added necessary documentation (if appropriate) ;

Short description of what this resolves:

Add support for Awaitable

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[jbevain]

Nice!