Add matrix-user-creator role - automatic user account creation support

We no longer ask users to create Matrix user accounts for these bots:

- Postmoogle
- Honoroit
- Reminder Bot

Other bots and services (matrix-registration-bot, maubot, mjolnir,
Dimension, etc.) require an Access Token to run (not a password),
so this new role doesn't help for them.

It does help for the above bots though, and for defining your own
"initial user accounts" in the `matrix_user_creator_users_additional`
variable.

roles/matrix-user-creator/defaults/main.yml

@@ -0,0 +1,22 @@
+---
+
+# matrix-user-creator is a role that aims to automate initial Matrix user account creation.
+#
+# This role only supports initial user account creation and will not manage subsequent user-type changes
+# or password changes.
+#
+# The playbook registers various bot user accounts automatically using this role by injecting
+# user creation definitions into the `matrix_user_creator_users_auto` variable.
+#
+# To get started creating your own Matrix user accounts, use the `matrix_user_creator_users_additional` variable.
+
+# matrix_user_creator_users_auto holds a list of users that should be created on the Matrix homeserver.
+# The playbook adds some user definitions here to have them end up in `matrix_user_creator_users` (see `vars/main.yml`) and get automatically created.
+# This value is influenced by the playbook and will be overwritten elsewhere (`group_vars/`, etc.)
+# To add your own user definitions, use the `matrix_user_creator_users_additional` variable.
+matrix_user_creator_users_auto: []
+
+# matrix_user_creator_users_additional holds a list of users that should be created on the Matrix homeserver.
+# Add your own users here to have them end up in `matrix_user_creator_users` (see `vars/main.yml`) and get automatically created.
+# For example syntax for this variable, see the documentation for `matrix_user_creator_users` in `vars/main.yml`.
+matrix_user_creator_users_additional: []