Add support for Google Artifact Registry (GAR) (#692)

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## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
The following PR:
* Deprecates the existing OCI reference parser
* Deprecates the existing OCI way of authenticating and accessing the
image manifest
* Replaces the above by leveraging go-containerregistry
* Introduces support for Google Artifact Registry
* Enables us to easily add support for other OCI-compliant registries in
the future
* Introduces better error handling
* Introduces custom context timeouts for validation 

## How Has This Been Tested?
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tested? -->

## Breaking Changes
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## Types of changes
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the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
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that apply. -->
- [ ] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [ ] My code follows the repository's style guidelines
- [ ] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
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Related to #427

---------

Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>

pkg/model/constants.go

@@ -13,8 +13,6 @@ const (
 const (
 	RegistryURLNPM    = "https://registry.npmjs.org"
 	RegistryURLPyPI   = "https://pypi.org"
-	RegistryURLDocker = "https://docker.io"
-	RegistryURLGHCR   = "https://ghcr.io"
 	RegistryURLNuGet  = "https://api.nuget.org"
 	RegistryURLGitHub = "https://github.com"
 	RegistryURLGitLab = "https://gitlab.com"