Add ability to list projects by group

Problem to solve

As a user I want to list all my repositories in a group So that I am aware what projects are in my group

Inspired by #1169 (closed)

Proposal

Add flag to glab project list --group group_name

(Alternative is to add a group command)

Links / references

Current help for project list command Note --repo should be removed as a repo is a project, so a project cannot have a list of projects

❯ glab project list --help
Get list of repositories.

USAGE
  glab repo list [flags]

ALIASES
  users

FLAGS
  -a, --all               List all projects on the instance
      --member            Only list projects which you are a member
  -m, --mine              Only list projects you own (default true)
  -o, --order string      Return repositories ordered by id, created_at, or other fields (default "last_activity_at")
  -p, --page int          Page number (default 1)
  -P, --per-page int      Number of items to list per page. (default 30)
  -R, --repo OWNER/REPO   Select another repository using the OWNER/REPO or `GROUP/NAMESPACE/REPO` format or full URL or git URL
  -s, --sort string       Return repositories sorted in asc or desc order
      --starred           Only list starred projects