Manage Rancher users, global roles, tokens and settings

Extend the yaml-discovery config to four more rancher2 kinds so Rancher
objects beyond the OIDC auth provider stop being unmanaged clickops.

- Discover config/users, config/roles, config/tokens and config/settings
  in config/config.hcl and pass them through terragrunt.hcl
- Add rancher2_user (password from Vault kv-v2) plus per-user
  rancher2_global_role_binding on "<username>/<role>" keys
- Add rancher2_global_role with dynamic rules blocks
- Add rancher2_token, documenting that the provider has no user selector
- Add rancher2_setting and seed config/settings/server-url.yaml
- Document every yaml schema in the README

config/settings/server-url.yaml

@@ -0,0 +1,3 @@
+# Rancher's server-url setting. Must match the URL Rancher is reached on,
+# otherwise newly registered cluster agents dial the wrong endpoint.
+value: https://rancher.k8s.syd1.au.unkin.net