API types for caches, exports, and platforms

[blampe]

This is a large change but it's mostly boilerplate for stronger typing around
Docker's stringy-typed CLI parameters, plus logic to convert our types into the
CSV format Docker expects.

For background, Docker accepts types like this:

--cache-from=type=local,src=/foo/bar --platform=linux/amd64 --output=type=registry,ref=docker.io/foo/bar:baz

We were accepting the same style of strings, but this is awkward to work with
once things become moderately complex. To remedy this, we add typing around
platforms, exports, cacheTo and cacheFrom:

• platforms: There are only a few dozen supported OS/Arch combinations, so
  this becomes an enum.

• cacheTo/cacheFrom/exports: These become union-ish types (I don't think
  go-provider allows us to create OneOf types). We preserve a "raw" option if
  the user would prefer to use Docker-style strings.

I'm bundling all of these together because the type signature changes will
require me to manually clean up state files in order to upgrade folks on the
alpha (refs. pulumi/pulumi-go-provider#193). Please
tell me if you have API feedback because this will be difficult to change!

If we're happy with the direction, I'll probably cut a beta from this branch.

Other API tweaks while I was in here:

• Tags are now only required if pushing to a registry, to match Docker.

• Multiple exports are no longer allowed to match pre-v25 Docker. After
  Upgrade to docker v25 #958 we should be able to
  remove this restriction.

• Expose the build target parameter in the SDK.

Fixes #970

[github-actions]

Does the PR have any schema changes?

Does the PR have any schema changes?

Looking good! No breaking changes found.

New resources:

• buildx/image.Image

Maintainer note: consult the runbook for dealing with any breaking changes.

[blampe]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• E2E buildx tests #996
• Support inline dockerfiles, named and remote contexts #1000
• API types for caches, exports, and platforms #993 👈
• Multi-platform caching for buildx #981
• Handle unknown inputs and buildOnPreview #980
• Share build context hashing #962
• buildx auth #948
• buildx.Image MVP #899
• Upgrade Docker SDK #898
• Initial go-provider integration #890
• feature/buildkit

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