Invite a person to join the workspace

Invites a person to the workspace by email. role defaults to TenantAdmin so they can manage the workspace immediately; any other workspace role works too (never a platform role). They receive an email and can accept by signing in with Google, Microsoft, GitHub, or a password, whether or not they already use InfoSlides elsewhere. Fails with EntitlementRequired when the plan's user limit (active members plus pending invitations) is already reached.