feat(create-app): add custom template support via GitHub template sources

[derrick-nuby]

Summary

This PR adds support for using a GitHub repository as a template source in @dhis2/create-app.
In addition to built-in templates (basic, react-router), users can now scaffold from public GitHub template repositories using shorthand or URL formats, including optional branch and subdirectory targeting.

What Changed

1. Added template source parsing and detection for GitHub specifiers.
2. Added template source resolver that supports:
   • built-in templates
   • remote GitHub templates cloned into a temporary directory
3. Updated create flow to use templateSource and ensure resolver cleanup in finally.
4. Added interactive flow option for Custom template from Git.
5. Updated create-app docs and command docs with usage examples.

Supported Template Formats

1. owner/repo
2. owner/repo#ref
3. owner/repo#ref:subdir
4. https://github.com/owner/repo
5. https://github.com/owner/repo#ref:subdir

Validation

Executed local and smoke tests for:

1. Existing built-in template flow (regression-safe behavior).
2. GitHub URL template source.
3. GitHub URL with branch ref.
4. GitHub URL with branch + subdirectory.
5. Shorthand owner/repo#ref.

Notes

1. Scope intentionally focuses on GitHub and public repositories.
2. Private repo auth UX and non-GitHub hosts are out of scope for this PR.
3. I did not include tests as this repo doesn't have a test suite, but I manually verified all scenarios and flows.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
1 Security Hotspot
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[kabaros]

Thanks a lot for this @derrick-nuby ! I left some comments here .. let me know if it's not clear, or you disagree

Thanks again for the great code, and taking initiative in adding this feature 🙏🏿

[kabaros]

maybe for built-in templates, just resolve here in this module - no need to create a map, pass it and resolve it .. the getTemplateDirectory (renamed to getBuiltInTemplateDirectory) already does most of that responsibility.

So something like

let resolvedTemplate = getBuiltInTemplateDirectory(selectedOptions.templateSource)

if(!resolvedTemplate) {
  // then do everything related to Git repos including cleanup
 // 
}

no need to pass the map to resolveTemplateSource or make it aware of built-in tempaltes. (maybe rename the module to resolveExternalTemplates)

[kabaros]

maybe better named validateTemplateDirectory?

• also could you move these helper methods into their own files (similar to isGitTemplateSource) .. a lot if happening in this file now

[kabaros]

is using a subdirectory a pattern that you, at HISP Rwanda, use? it just seems like it adds a lot of complexity to the code, and I feel like it's more than enough to support the other three other ways of defining repos