This forks the distribution's registry client auth-challenge code v3.0.0;
distribution/distribution@9ed95e7

The client code has moved to an internal package in the distribution project,
and is used to handle `WWW-Authenticate` headers. It was written a long time
ago, and only handles RFC 2617 (superseded by RFC 9110). Possibly there's more
current implementations for this, but this would introduce new dependencies, so
we can keep the status quo for now, and fork this code as an internal package.

Keeping it internal, due to the known limitations mentioned above, and to avoid
it being used by others assuming it's a general-purpose implementation.

This is the result of the following steps:

    # install filter-repo (https://github.com/newren/git-filter-repo/blob/main/INSTALL.md)
    brew install git-filter-repo

    # create a temporary clone of docker
    cd ~/Projects
    git clone https://github.com/distribution/distribution.git distribution_auth
    cd distribution_auth

    # switch to v3.0.0 (git-filter doesn't like multiple branches, so reset)
    git reset --hard v3.0.0

    # commit taken from
    git rev-parse --verify HEAD
    9ed95e7365224025ee89365e12cf128e1f1bf965

    git filter-repo --analyze

    # remove all code, except for 'internal/client/auth/challenge'
    # rename to 'pkg/v1/remote/internal/authchallenge'
    # include history of old location ('registry/client/auth/challenge')
    git filter-repo \
      --force \
      --path 'internal/client/auth/challenge' \
      --path 'registry/client/auth/challenge' \
      --path-rename internal/client/auth/challenge:pkg/v1/remote/internal/authchallenge

    # go to the target github.com/docker/docker repository
    cd ~/go/src/github.com/google/go-containerregistry

    # create a branch to work with
    git checkout -b fork_registryclient_authchallenge

    # add the temporary repository as an upstream and make sure it's up-to-date
    git remote add distribution_auth ~/Projects/distribution_auth
    git fetch distribution_auth

    # merge the upstream code
    git merge --allow-unrelated-histories --signoff -S distribution_auth/main

History was squashed:

- Refactor authorization challenges to its own package
  Split challenges into its own package. Avoids possible
  import cycle with challenges from client.
- Fix minor "Challanges" typo
- Fix gometalint errors
- Fix typo in NewSimpleManager() documentation
- Initial repo commit
- init
- format code with gofumpt

Move registry client internal

Our registry client is not currently in a good place to be used as the
reference OCI Distribution client implementation. But the registry proxy
currently depends on it. Make the registry client internal to the
distribution application to remove it from the API surface area (and any
implied compatibility promises) of distribution/v3@v3.0.0 without
breaking the proxy.

cleanup: move init funcs to the top of the source

We make sure they're not hiding at the bottom or in the middle
which makes debugging an utter nightmare!

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>