Conformance test redesign

[sudo-bmitch]

This is a redesign of the conformance tests with the following goals:

• Run as a command instead of a go test. This will allow unit testing and avoid some of the Go testing wrappers.
• Make the API testing more granular.
• Design the tests to run against different types of input data, including the various updates to image-spec.
• Minimize dependencies to avoid upstream breaking changes.
• Where possible, note unsupported APIs and fallback to alternatives.

This is still in draft for now, with this open PR for better visibility. Various remaining tasks include:

• [ ] Sparse index tests.
• [x] Edge cases on blob pushes (out of order, chunked push with data in the final put).
• [x] Pushing content with the wrong digest or an invalid digest algorithm.
• [ ] Verify all allowed HTTP status codes are appropriately handled.
• [ ] Cache/reuse auth tokens between HTTP requests.
• [ ] Rename conformance2 to conformance and delete the old conformance tests.
• [ ] Update the GitHub actions to test against multiple registries, and consider if/when failing 3rd party registries should block a merge to the distribution-spec.
• [ ] Generate an image in GitHub actions to be used by a conformance action.
• [ ] Update the conformance action.
• [ ] Update the oci-conformance repo to handle the new input (results will likely be summarized with two columns, API and Data, with green or gray status to show passing tests or unsupported).
• [ ] Add some unit tests.

Other wish list items:

• [ ] Add ability to run a single test.
• [ ] Add separate tests for foreign layers (URL defined) from non-distributable layers (specific media type).

Merging this will unblock #543. Closes #548. Closes #501. Closes #416.