Conforming with PEP 518 to use pyproject.toml instead of requirements.txt/setuptools

### Describe your idea/feature/enhancement

Provide a clear description. Ex. I wish SAM CLI would [...]

I wish SAM CLI would utilize pyproject.toml for requirements. Subsequently, this would use additional package managers like poetry to in the `sam build` process.

### Proposal

Add details of how to add this to the product.

Add functionality to read pyproject.toml when running `sam build`

Things to consider:
1. Will this require any updates to the [SAM Spec](https://github.com/awslabs/serverless-application-model)

This would likely add another way to setup code architecture and application requirements in template.yaml files.

### Additional Details