add terraform lambda layers sample + migrate serverless sample to new dir

[HarshCasper]

This PR:

• Migrates the serverless-lambda-layers sample to lambda-layers directory
• Add a new terraform lambda layers sample, forked from @joe4dev's example — https://github.com/joe4dev/localstack-lambda-layer-python-terraform-sample

TODO

• [ ] Add @joe4dev as the commit author before merge.