Allow replacing core plugins

[davidfestal]

Description

This PR allows replacing core Backstage features like scaffolder, catalog, or search, in order to customize them.

• For frontend plugins, this is simply done by wiring your own dynamic plugin to a dynamic route with the same path as a standard one (/ for the home page, or catalog for example)
• For backend plugins, before installing your own backend plugin replacement, you first have to disable the standard backend plugins statically linked into the backend application with one of the following environment variables:
  • DISABLE_STANDARD_CATALOG=true
  • DISABLE_STANDARD_SCAFFOLDER=true
  • DISABLE_STANDARD_SEARCH=true

Which issue(s) does this PR fix

• Fixes issue https://issues.redhat.com/browse/RHIDP-1500

PR acceptance criteria

Please make sure that the following steps are complete:

• GitHub Actions are completed and successful
• Unit Tests are updated and passing
• E2E Tests are updated and passing
• Documentation is updated if necessary (requirement for new features)
• Add a screenshot if the change is UX/UI related

How to test changes / Special notes to the reviewer

[openshift-ci]

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[github-actions]

The image is available at: quay.io/janus-idp/backstage-showcase:pr-1256!

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[github-actions]

The image is available at: quay.io/janus-idp/backstage-showcase:pr-1256!

[openshift-merge-robot]

PR needs rebase.

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