ECR: Add ability to set tag mutability on repo creation

[dvan-sqsp]

Hi!

We have a use-case where we want to force our ECR repos to have tag mutability set to IMMUTABLE. Since we have repo's with create_repository set to true, we only want to modify ones that are managed by the drone plugin.

AWS ECR tag mutability is a string of MUTABLE or IMMUTABLE. To simplify usage, I decided to go with a boolean to avoid typos on the plugin usage side. By default, tag_immutable is false, thus keeping existing behavior. Depending on the boolean, the code will use the provided SDK constants for MUTABLE and IMMUTABLE

usage example:

steps:
  - name: push-image
    image: plugins/ecr
    settings:
      ...
      create_repository: true
      tag_immutable: true