[FEATURE] Multi-account basic template - move CICD infra to separate repo & add pipeline self-mutation

[kukushking]

Is your feature request related to a problem? Please describe.
Current multi-account basic template has a static CodePipeline that delivers endpoints to dev/test/prod. The pipeline is currently built into the Service Catalog template and does not allow customisation after it's been created.

Describe the solution you'd like

• Move CodePipeline into a separate {project_name}-infra repository (in addition to -build and -deploy repos created by the template).
• Enable self-mutation on the pipeline.