Fixes issue

Various requests return multiple warning messages if a registry outputs a warning header on every request.

Describe the change

When a registry returns a warning header on every request, clients should try to consolidate these.

How to verify it

Test with olareg:

olareg serve --store-type mem --warning "Ephemeral registry, content may be deleted"

Requests with regctl to that registry should return one warning per command.

Changelog text

• Feat: Consolidate warnings.

Please verify and check that the pull request fulfills the following requirements

Testing this would need something monitoring stderr for every test, and enabling a registry with warnings on every test. For now, this is just a best effort.

• Tests have been added or not applicable
• Documentation has been added, updated, or not applicable
• Changes have been rebased to main
• Multiple commits to the same code have been squashed