bake: initial set of composable bake attributes

[jsternberg]

This allows using either the csv syntax or object syntax to specify
certain attributes.

This applies to the following fields:

• output
• cache-from
• cache-to
• secret
• ssh

There are still some remaining fields to translate. Specifically
ulimits, annotations, and attest.

Related to #438.

[jsternberg]

Very incomplete first draft. This requires some changes to the gohcl package in the hcl package for it to work. The biggest thing is overriding the ImpliedType method and copying the full implementation from go-cty so that we can customize the behavior for certain types. In this case, I create a FromCtyValue interface and that works for our purposes.

The only changes to gohcl is creating a DecodeOptions which allows us to inject our own version of ImpliedType into the decoder. I also made it so we can change convert, but modifying the convert implementation was a lot more involved than modifying ImpliedType and using the existing capsule type.

[jsternberg]

I've modified this to include a small fork of the gohcl package temporarily while we resolve any legal stuff getting the CLA signed for upstream.

[crazy-max]

I've modified this to include a small fork of the gohcl package temporarily while we resolve any legal stuff getting the CLA signed for upstream.

https://github.com/docker/buildx/blob/master/bake/hclparser/merged.go is also a fork from upstream. Maybe we could move it to an hcl package under hclparser one and then move LICENSE and gohcl in hcl?

./bake/
├── ...
├── hclparser
│   ├── ...
│   ├── hcl
│   │   ├── LICENSE
│   │   ├── gohcl
│   │   └── merged.go
│   ├── ...
└── ...

[jsternberg]

This PR is becoming fairly large. There are a few more attributes that have to be migrated to work, but it might be good to review and merge what we have to land this feature. I'm changing this to ready to review while I work on the remaining attributes.