Allow other principals

[Ikana]

Describe the Feature

Allow for other principals other than AWS for access control.

Expected Behavior

Allow Service or other values in principals_readonly_access.

Use Case

For letting services like lambda pull from the repository.

Describe Ideal Solution

To specify the type of principals when the value is not AWS, you could also specify the whole access policy.

Alternatives Considered

Using other modules.

[davidkarlsen]

@Ikana lambda is a bit strange - that it requires the permissions to be embedded in the resource (the ECR-repo) - have you found any workaround to that? It seems it's not possible to place the permission in a separate iam-role?
CC @kjetiloen

[dsme94]

#88 will allow for cross-account lambda access.