Add option to specify lambda via image_uri

[nmnielsen]

Are there any dependencies (software or human) that need to be addressed before this PR is merged?

What ticket(s) or other PRs does this relate to?

What was the problem or feature?

Want to be able to create a Lambda based on an image in ECR.

What was the solution?

Add the image_uri option to the variables and to the creation of the Lambda resource. Additionally add verifications and conditionals to ensure that we don't specify conflicting S3 and ECR configurations. (this commit, we can add these once we get to Terraform > 1.2.0)

• Security Impact has been considered (if yes please describe): none
• Network Impacts have been considered (if yes please describe): none

Where does this work fall on the Good - Fast spectrum?

Any additional work needed as a result of merging this PR (deploy steps, other PRs, etc.)?

[nmnielsen]

Alright! That took more iterations than I expected - should be ready to review now!