Add initial support for external build modules

[myovchev]

Summary

NOTE: Internal PR to a Feature Branch.

Allow Apostrophe to call external build module (initial setup).

What are the specific steps to test this change?

1. Register external module
2. Register duplicate external modules
3. Proxy the build to external module if available

What kind of change does this PR introduce?

(Check at least one)

• Bug fix
• New feature
• Refactor
• Documentation
• Build-related changes
• Other

Make sure the PR fulfills these requirements:

• It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• The changelog is updated
• Related documentation has been updated
• Related tests have been updated

If adding a new feature without an already open issue, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[linear]

PRO-6641 [Initial Core Changes] Feature Delegate Build to an external module (core)