feat: support tag sub command

[chlins]

This pull request introduces a new tag command to the CLI and updates the backend to support tagging model artifacts. Additionally, it modifies the Makefile to handle the installation of mockery using go install instead of Homebrew. Below are the most important changes:

Closes: #89

New tag Command:

• cmd/root.go: Added tagCmd to the root command.
• cmd/tag.go: Introduced the tag command implementation, which includes the command definition and the runTag function to handle the tagging logic.

Backend Updates:

• pkg/backend/backend.go: Added the Tag method to the Backend interface.
• pkg/backend/tag.go: Implemented the Tag method to create a new tag that refers to the source model artifact.
• pkg/backend/tag_test.go: Added unit tests for the Tag method to ensure its correct functionality.

Storage Interface Enhancements:

• pkg/storage/storage.go: Updated the Storage interface to include the MountBlob method.
• pkg/storage/distribution/distribution.go: Implemented the MountBlob method to mount blobs in the storage.

Mockery Installation Update:

• Makefile: Changed the check-mockery and install-mockery targets to use go install for installing mockery instead of Homebrew, and introduced the MOCKERY_VERSION variable. [1] [2]

[gaius-qi]

LGTM