add ImageIdentifier.repository for custom-registry image lookups

[dansola]

Why are the changes needed?

When a flyte v2 image is built with a custom registry override (clone(registry=...)), the
remote-build cache never hits — every deploy rebuilds an image that's already sitting in ECR.

The reason: the existence check (ImageService.GetImage) only receives the image name, so the
dataplane handler always looks in the cluster's default repository, while the build pushed the
image to a different repo. There's currently no way to tell the backend which repository an
image actually lives in, so it can never find custom-registry images and rebuilds them every time.

This PR adds that missing piece of information to the API. (Internal ref: SE-775.)

What changes were proposed in this pull request?

Adds an optional repository field to ImageIdentifier:

message ImageIdentifier {
  string name = 1 [(buf.validate.field).string.min_len = 1];

  // Optional registry/repository the image lives in, e.g.
  // "<account>.dkr.ecr.<region>.amazonaws.com/<repo>" for an image built with a custom registry
  // override (clone(registry=...)). When set, GetImage looks the image up there; when empty, the
  // cluster's default repository is used.
  string repository = 2;
}

It's backwards-compatible — an empty repository preserves today's default-repo behavior. This is
the foundation of a 3-part change: the SDK sets this field on the GetImage request, and the
dataplane GetImage handler uses it to look the image up in the right repo (see Related PRs).

Includes the regenerated Go / Python / swagger stubs.

## How was this patch tested?

NA

### Labels

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- **fixed**: For any bug fixed.
- **security**: In case of vulnerabilities

This is important to improve the readability of release notes.

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## Check all the applicable boxes <!-- Follow the above conventions to check the box -->

- [ ] I updated the documentation accordingly.
- [ ] All new and existing tests passed.
- [ ] All commits are signed-off.

## Related PRs

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## Docs link

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[dansola]

https://github.com/unionai/cloud/pull/16338
flyteorg/flyte-sdk#1163

[pingsutw]

nit

Suggested change

	string repository = 2;
	string registry = 2;

[EngHabu]

Is the idea that we will concat these values to create the final image url to check on?

If that's the case, we do need registry, repo and I'm assuming name already has the version?