tg beta clusters create and update add flags for Headlamp and Slurm Web add-ons

tg beta clusters create and tg beta clusters update now expose flags for Headlamp and Slurm Web cluster add-ons.

• Create: Pass --headlamp-addon or --slurm-web-addon to enable an add-on at cluster creation.
• Update: Pass --headlamp / --no-headlamp or --slurm-web / --no-slurm-web to toggle add-ons on an existing cluster.

See the clusters CLI reference.