Add admin hoted app local spec

[alfonso-noriega]

WHY are these changes introduced?

The admin extension type was previously hardcoded as a string literal, which prevented proper specification handling and caused inconsistencies in the extension system.

WHAT is this pull request doing?

• Creates a new AdminSpecIdentifier constant and corresponding specification file for admin extensions
• Replaces the hardcoded 'admin' string with the proper AdminSpecIdentifier in the configuration extension IDs array
• Adds the admin specification to the loaded specifications list
• Implements admin extension as a contract-based module with single UID strategy and copy files build mode
• Adds a default case to the extension instance UID generation logic
• Updates the app configuration schema contribution logic to handle specifications without shape definitions
• Adds uidStrategy parameter support to contract-based module specifications

How to test your changes?

1. Create an admin extension in a Shopify app
2. Verify the extension builds and deploys correctly
3. Confirm the extension receives a proper UID based on its specification identifier
4. Test that the hosted app copy files step executes during deployment

Measuring impact

How do we know this change was effective? Please choose one:

• n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix
• Existing analytics will cater for this addition
• PR includes analytics changes to measure impact

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes

[alfonso-noriega]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Add admin hoted app local spec #7027 👈 (View in Graphite)
• Add manifest generation to include_assets build step #7016 : 1 other dependent PR (#7017 )
• Refactor extension build process to use modular build steps #6867 : 2 other dependent PRs (#6868 , #6869 )
• Add client steps implementation to support existing functionalities #6966 : 1 other dependent PR (#6941 )
• Create abstract client steps infrastracture to externalize the ap modules client configurations #6965
• Clean up module outputFile calculation and move it to the specifications #6967
• main

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