feat: add integration tests stack

[nmoutschen]

Issue #, if available: #377

Description of changes:

Adds a SAM template and AWS Lambda functions to test the crates in this repository on AWS. This allows us to test Rust functions and extensions on both AL1 and AL2. Furthermore, I've included a sample Lambda function in Python so we can test extensions on non-provided runtimes as well.

I'm not a big fan of defining a new crate for those tests, but I don't see a better way to do it at the moment.

• lambda_runtime with handler_fn
• lambda_runtime with Handler trait
• lambda_http with handler
• lambda_extension with extension_fn
• lambda_extension with Extension trait

By submitting this pull request

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I confirm that I've made a best effort attempt to update all relevant documentation.

[coltonweaver]

Just to gather motivations on this - it's essentially setting up an automated way of uploading a set of functions based on this library to actual Lambda and validating that we're able to invoke them successfully? Sort of a pre-PR check? If so, great call. I love this idea.

Edit: Just took a look at your issue assigned to this. Completely agree with you motivation. I think this is a great starting place. Approving. Merge when you want.

[nmoutschen]

I've had to disable the multiple_crate_versions clippy lint rule, as time uses itoa-0.4.8 and serde_json uses itoa-1.0.1. This will also fix the issue with #382