Request: Add support for user management

[KNiepok]

For more populous orgs it would be great to have a programmatic approach to manage users and group access when utilizing the new user management beta.

I propose introducing new data sources: spacelift_managed_user - representing our graphQL ManagedUser spacelift_managed_user_group - representing our graphQL ManagedUserGroup, which is actually an IDP group mapping, so maybe more clear naming can be invented

and resources: spacelift_managed_user - this might be a bit tricky since we are creating a user while sending an invitation at the same time, so either this is a single resource (see graphQL managedUserInvite mutation); that does conditional re-invitation in the case the managed user didn't accept the invitation yet but his credentials change; or we can create a separate resource for managing invitations (we have an explicit managedUserResendInvite mutation). spacelift_managed_user_group - this one should be more straightforward, it's a regular CRUD for graphQL ManagedUserGroupIntegration

[michieldewilde]

@KNiepok We can close this because #474 and #473 are done.