Feature: user groups

[jiparis]

We want to implement user groups. The Org Admin would be able to:

• Create and manage groups groups and members.
  • CRUD operations on groups
  • Add or remove people to groups

People added to groups can be "maintainers", so they can manage group members (and add other maintainers).

CLI examples (get inspiration from chainloop org member commands:

// ORG LEVEL (by org admin/owner)
chainloop group create --name foo

// by org admin/owner or group maintainer
chainloop group member add --name foo --user foo@chainloop.dev --maintainer

• if user doesn't exist in Chainloop yet, it should send an invitation to join the organization. On acceptance, it would add the user as member of the group
• The same would happen to projects.