[chore] Split core and contrib workflows

[TylerHelmuth]

This PR separates the Core and Contrib distributions into separate workflows.

The repository continues to use a single goreleaser.yaml file because:

1. we already have a process to generate it
2. goreleaser has features to support a single file

We specifically take advantage of id to specify exactly which things in the goreleaser.yaml file get built.

For now I did not change the triggers - both sets of workflows will currently always be triggered at the same time, like they are today.

Related to #429

Example workflows:

• https://github.com/TylerHelmuth/opentelemetry-collector-releases/actions/runs/7908747507
• https://github.com/TylerHelmuth/opentelemetry-collector-releases/actions/runs/7908747499

I couldn't test the actual release bc I don't have a goreleaser-pro key.

[TylerHelmuth]

Created #478 to take care of ensuring partial is included in the generate.

[jpkrohling]

It looks mostly right, but there are a few cosign-related changes that seem misplaced.

[TylerHelmuth]

@jpkrohling all those downgrades were the result of a bad merge. Should now be fixed.