Add union data resolver

[begelundmuller]

Adds a union resolver that invokes multiple sub-resolvers and returns the union of their results and schemas. Example use for partitions:

type: model

partitions:
  union:
    - glob:
        path: s3://bucket/year=*/month=*
    - glob:
        path: s3://bucket/year=*/month=*/day=*
        last: 7

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!