postgres: Optionally include pgdg apt repo

postgres/defaults/main.yml

+---
+
+postgres_pgdg_repo: false

postgres/tasks/main.yml

 ---
 
+- when: postgres_pgdg_repo
+  block:
+    - name: Install pgdg apt repo key
+      apt_key:
+        url: https://www.postgresql.org/media/keys/ACCC4CF8.asc
+        state: present
+    - name: Configure pgdg apt repo
+      apt_repository:
+        repo: >-
+          deb
+          https://apt.postgresql.org/pub/repos/apt
+          {{ ansible_distribution_release }}-pgdg
+          main
+
 - name: ensure postgres packages are installed
   apt:
     name:
+      # yamllint disable-line rule:line-length
       - postgresql{{ "-{}".format(postgres_version) if postgres_version is defined }}
       - python-psycopg2
       - python3-psycopg2