Add OpenAPI Python3 example

[spitzzz]

Issue #, if available:

Not applicable

Description of changes:

Added an example for using an OpenAPI template to deploy a basic API. For this example I utilized the example lambda function from hello-world-python3 to incorporate referencing a lambda into the example template.

Notes to Reviewer(s)

• I tested deploying this code and invoking the API successfully, feel free to replicate this on your own. For your convenience you can follow these steps:

sam build
sam deploy --guided

When using the sam deploy --guided step make sure you pass in values for the required parameters.

PR Required Notice

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[spitzzz]

@carvantes @ylynn @mlaff Would it be possible to get a review on this? I think this example could add value - particularly because it seems the old Swagger examples were removed.