Support for lambda source from S3

[rafaelfelix]

this patch enables the module to allow defining the lambda function code from an s3_bucket and key, instead of locally.

[raymondbutcher]

Unfortunately I think this needs an s3 variation for all Lambda function resources, not just the first one.

• Done:
  • lambda_s3
• Missing:
  • lambda_with_dl_s3
  • lambda_with_vpc_s3
  • lambda_with_dl_and_vpc_s3

(not sure about those names)

The main purpose of this module is to handle the Lambda packaging process for you. I'm not sure if I would use this module if using pre-built packages. I suppose the IAM policy resources are still somewhat useful.

So is it worth adding 4 copies of the Lambda resource to get this feature? I'm not sure.

[rafaelfelix]

@raymondbutcher thanks for taking the time and reviewing this PR. The reason why this change is useful to me is, I want to manage the creation of the lambda resources using terraform (and your module does that quite nicely), while managing the deployment of the actual code from a CI/CD pipeline (as it fits better in the workflow my team is used to). For that purpose, fetching the source code from S3 is the only way to keep consistency when applying terraform by redeploying the last version of the artifact created by the pipeline.

Now, given the motivation for the change I'm proposing (sorry not to have done this before, should have been more clear about it in the PR description), I can create these variations you requested in order to get it through. Shall I do it?