Ability to retrieve image IDs when using multiple exporters

Description

If I invoke docker buildx build --output type=docker --output type=oci,dest=oci.tar contextPath I should be able to add --iidfile or --metadata-file and have the generated files contain the image IDs of both images.

If you try this today, the generated files will contain one of the IDs but not the other.

I am expecting the generated files to contain both IDs and an indication of which exporter each one corresponds to.

Today, if I invoke the above command on this Dockerfile:

FROM scratch

COPY empty.txt .

I get this output:

[internal] load build definition from Dockerfile
	transferring 76/0 0.007
[internal] load .dockerignore
[internal] load build context
	transferring 2512/0 0.004
[1/1] COPY logback-test.xml .
exporting to oci image format
	exporting layers 0/0 0.058
	exporting manifest sha256:56b029f44ccd04d2f15a2b9ad39957f469c29fc880053750c2913ee09aab64bd 0/0 0.735
	exporting config sha256:fcbc65ed4feab765a528cdaa54b355b433dc6e4d86f29cc6dfc0a4ca20315110 0/0 0.004
	sending tarball 0/0 0.006
exporting to image
	exporting attestation manifest sha256:28f376eaa0ec7b68391b720540d177f6a1caacb59b020bdc8bec92c91845688f 0/0 0.018
	exporting manifest list sha256:3f4a3acab60068864e10ec341ef83a418edfd355185bd70d792be8c719eccc51 0/0 0.01
	naming to moby-dangling@sha256:3f4a3acab60068864e10ec341ef83a418edfd355185bd70d792be8c719eccc51 0/0 0.003
	unpacking to moby-dangling@sha256:3f4a3acab60068864e10ec341ef83a418edfd355185bd70d792be8c719eccc51 0/0 0.018

The OCI and Docker images have different IDs. I'd like to be able to look then up without parsing the console output.

Start at the docker buildx build handling for --iidfile and --metadata-file, using the multiple-exporter command in the issue as the repro. Trace how each exporter produces its image ID and define metadata that records both IDs with their exporter identities; done when the generated files expose both IDs without parsing console output.