Add tailscale_tailnet_membership resource for user and membership management

[pmpaulino]

Is your feature request related to a problem? Please describe.
There’s no Terraform resource for managing who is in the tailnet. I can’t add/invite users, disable or re-enable them, or remove them from Terraform, only via the API or admin UI.

Describe the solution you'd like
A resource (e.g. tailscale_tailnet_membership) that lets me ensure an identity is in the tailnet (sending an invite when needed), set role (member/admin), suspend/restore, and remove membership. Idempotent create when the user or invite already exists, and optional “downgrade on destroy” (downgrade or suspend instead of delete). Very much influenced by the pattern/idea of the GitHub provider membership resource.

Additional context
I’ve already done a POC of this and would be happy to own the implementation and open a PR.