build: opt to set progress warnings in response

[crazy-max]

alternative to #2511 and similar to #2280, we can set build checks warnings in the metadata file. Does not set them by default but can be enabled by using the BUILDX_METADATA_WARNINGS=1 environment variable.

This will be useful with our GitHub Actions so we can generate GitHub Annotations without having to do a subrequest build like in https://github.com/crazy-max/docker-build-push-action/pull/39/files#diff-4fab5baaca5c14d2de62d8d2fceef376ddddcc8e9509d86cfa5643f51b89ce3dR132

@tonistiigi Also added extra commit to dedup warnings for bake related to #2534 (comment)

# syntax=docker/dockerfile-upstream:master
frOM busybox as base
cOpy Dockerfile .

from scratch
COPy --from=base \
  /Dockerfile \
  /

$ BUILDX_METADATA_WARNINGS=1 docker buildx build . --metadata-file md.json
...
#12 exporting to image
#12 exporting layers done
#12 writing image sha256:d0bb2193d26744307a7003c5397e277d1fa73ee979851b0c2ed2afad89d2e44b done
#12 DONE 0.0s

#13 resolving provenance for metadata file
#13 DONE 0.0s

 3 warnings found (use --debug to expand):
 - ConsistentInstructionCasing: Command 'frOM' should be consistently cased (line 2)
 - ConsistentInstructionCasing: Command 'cOpy' should be consistently cased (line 3)
 - ConsistentInstructionCasing: Command 'COPy' should be consistently cased (line 6)

View build details: docker-desktop://dashboard/build/default/default/wuiiyvfzfof6q93b69sp97n2l

md.json

{
  "buildx.build.provenance": {
    "buildType": "https://mobyproject.org/buildkit@v1",
    "materials": [
      {
        "uri": "pkg:docker/docker/dockerfile-upstream@master",
        "digest": {
          "sha256": "c2da757515621f3767427a157f351149ff48a69e0550afa08fa768feaaf466c7"
        }
      },
      {
        "uri": "pkg:docker/busybox@latest?platform=linux%2Famd64",
        "digest": {
          "sha256": "9ae97d36d26566ff84e8893c64a6dc4fe8ca6d1144bf5b87b2b85a32def253c7"
        }
      }
    ],
    "invocation": {
      "configSource": {
        "entryPoint": "Dockerfile"
      },
      "parameters": {
        "frontend": "gateway.v0",
        "args": {
          "cmdline": "docker/dockerfile-upstream:master",
          "source": "docker/dockerfile-upstream:master"
        },
        "locals": [
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          },
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          }
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        "platform": "linux/amd64"
      }
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