Allow Harbor integration to connect to multiple projects on Harbor

Proposal

Currently the Harbor integration only allows one to select a single project on harbor.

The customer I'm representing uses Harbor as a container registry and they use system-account for 1 user for covering the following Proxy-Cache Projects:

• Docker Hub
• GHCR - GitHub Container Registry
• MCR - Microsft Container Registry
• Quay
• A Public-Release Internal Project

They propose making the Harbor integration accept multiple projects, perhaps as a comma-seperated list:

"docker-hub,ghcr,mcr,quay,public-releases"

This will allow them to use it system wide.

I understand that Project-Robot-Accounts in Harbor only have access to 1 Project, but System-Robot account can give access to many projects, see here: https://goharbor.io/docs/2.12.0/administration/robot-accounts/

So this should be possible from Harbor's perspective.

Referring Zendesk Ticket (GitLab internal): https://gitlab.zendesk.com/agent/tickets/619723