Add ignore_image_cache config

[mwaskom]

Describe your changes

Adds a new field to the config, ignore_image_cache. This will be included in the ImageGetOrCreateRequest. It will effectively ignore the Image cache (in practice, it will include the App ID and version in the cache key). This can be useful for testing whether Apps can run when rebuilding an Image from top-to-bottom without then forcing other Apps that depend on the same base layer(s) to rebuild.

Backward/forward compatibility checks

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Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself

Note on protobuf: protobuf message changes in one place may have impact to
multiple entities (client, server, worker, database). See points above.

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Changelog

• Added a new config field, ignore_image_cache, which will cause Images used by the App to rebuild without clobbering any existing cached Images. This can be useful for testing an App's robustness to Image rebuilds without affecting other Apps that depend on the same base Image layer(s).