Make explorer package independently deployable and configurable

[stanlou]

The explorer package has limitations that prevent users from easily customizing and deploying it:

• Updates to the explorer require framework releases, creating a tight coupling.
• Users cannot easily change the explorer's appearance, behavior, or features.

Proposed Solution

Make the explorer a standalone project that:

• Move the explorer to its own repository.
• Allow explorer updates independent of framework releases.
• Create a Dockerfile for the explorer package and set up GitHub Actions workflow to build and push explorer images to github registry.
• Configure the CLI to pull and run the explorer Docker image via protokit explorer start command.
• Allow users to specify custom explorer images via CLI flag.