cli/config/types: make AuthConfig an alias for registry.AuthConfig

[thaJeztah]

• relates to https://github.com/docker/cli/pull/1642
• relates to https://github.com/moby/buildkit/pull/800

The AuthConfig type was forked from the docker/docker API types in commit 27b2797f7deb3ca5b7f80371d825113deb1faca1 to reduce the dependency on the docker API types in BuildKit and Buildx (see buildkit#800). Now that the API is a separate module with minimal dependencies, this should no longer be a big concern, so this patch un-forks the type.

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[codecov-commenter]

Codecov Report

:x: Patch coverage is 78.57143% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cli/command/registry.go	35.71%	9 Missing :warning:
cli/config/memorystore/store.go	71.42%	2 Missing :warning:
cli/command/container/auth_config_utils.go	0.00%	1 Missing :warning:

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