Weco-deploy

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Fix wait-for-deploy by restoring cache-busting ability to ECS service fetcher

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Simplify the code slightly by removing account/registry ID overrides, which we never use in practice. We always use the default ECR registry.

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Add some models to describe the weco-deploy config. These models aren't used yet, but they should make it easier to work on weco-deploy in future.

Note: this release adds a dependency on ``attrs`` and ``cattrs`` (which provide similar typed-models-to-JSON-and-back functionality to case classes and Circe in our Scala apps).

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Simplify the code slightly by removing per-image repository and per-service overrides, which we never use in practice.

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More refactoring towards support for ECR Public.

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Fixes running prepare when there are no releases

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