add postgresql role for ens. psql and opt. some dbs and users are set up

Add postgresql role for ensuring postgresql is installed.
Furthermore the role optionally takes some basic configuration to ensure
databases with their owners and users are set up as specified.

This is a requirement for a new netbox role.

roles/postgresql/README.md

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+# Role `postgresql`
+
+Ensures `postgresql` is installed by installing the distributions package.  
+Also ensures the optionally given databases and users are set up as specified.
+
+## Supported Distributions
+
+Should work on Debian-based distributions.
+
+## Required Arguments
+
+None.
+
+## Optional Arguments
+
+- `postgresql__dbs`: List of databases with their owner to ensure are set up.  
+- `postgresql__dbs.*.name`: Name of the database.
+- `postgresql__dbs.*.owner`: Owner of the database.
+- `postgresql__users`: List of users to ensure are set up.
+- `postgresql__users.*.name`: Name of the user.
+- `postgresql__users.*.password`: Optional password for the user.
+   If left unset, the user will have no password set, but can still connect using [peer authentication](https://www.postgresql.org/docs/current/auth-peer.html) on the local system.
+   (Peer authentication works when a password is set as well.)
+
+## Example Arguments
+
+```yaml
+postgresql__dbs:
+  - name: netbox
+    owner: netbox
+  - name: foo
+    owner: bar
+postgresql__users:
+  - name: netbox
+    password: super_secret
+  - name: bar
+```

roles/postgresql/defaults/main.yaml

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+postgresql__dbs: [ ]
+postgresql__users: [ ]

roles/postgresql/meta/argument_specs.yaml

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+argument_specs:
+  main:
+    options:
+      postgresql__dbs:
+        type: list
+        elements: dict
+        required: false
+        default: [ ]
+        options:
+          name:
+            type: str
+            required: true
+          owner:
+            type: str
+            required: true
+      postgresql__users:
+        type: list
+        elements: dict
+        required: false
+        default: [ ]
+        options:
+          name:
+            type: str
+            required: true
+          password:
+            type: str
+            required: false
+            default: ""

roles/postgresql/tasks/main.yaml

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+- name: Ensure postgresql is installed
+  ansible.builtin.apt:
+    name:
+      - postgresql
+  become: true
+
+- name: Ensure Python library for community.postgresql is installed if needed
+  ansible.builtin.apt:
+    name:
+      - python3-psycopg
+  become: true
+  when: postgresql__dbs != [ ] or postgresql__users != [ ]
+
+- name: Ensure users
+  community.postgresql.postgresql_user:
+    name: "{{ item.name }}"
+    password: "{{ item.password | default('') }}"
+  become: true
+  become_user: postgres
+  loop: "{{ postgresql__users }}"
+
+- name: Ensure dbs with owners
+  community.postgresql.postgresql_db:
+    name: "{{ item.name }}"
+    owner: "{{ item.owner }}"
+  become: true
+  become_user: postgres
+  loop: "{{ postgresql__dbs }}"