feat(cnpg): add postgresql section for custom config (#344)

**Description**
As requested by @stavros-k allow custom postgresql config on a per-chart
basis

**⚙️ Type of change**

- [x] ⚙️ Feature/App addition
- [ ] 🪛 Bugfix
- [ ] ⚠️ Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- [ ] 🔃 Refactor of current code

**🧪 How Has This Been Tested?**
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**📃 Notes:**
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**✔️ Checklist:**

- [x] ⚖️ My code follows the style guidelines of this project
- [x] 👀 I have performed a self-review of my own code
- [x] #️⃣ I have commented my code, particularly in hard-to-understand
areas
- [x] 📄 I have made corresponding changes to the documentation
- [x] ⚠️ My changes generate no new warnings
- [x] 🧪 I have added tests to this description that prove my fix is
effective or that my feature works
- [x] ⬆️ I increased versions for any altered app according to semantic
versioning

**➕ App addition**

If this PR is an app addition please make sure you have done the
following.

- [x] 🪞 I have opened a PR on
[truecharts/containers](https://github.com/truecharts/containers) adding
the container to TrueCharts mirror repo.
- [x] 🖼️ I have added an icon in the Chart's root directory called
`icon.png`

---

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---------

Co-authored-by: Stavros kois <s.kois@outlook.com>

library/common-test/tests/cnpg/custom_conf_test.yaml

@@ -0,0 +1,29 @@
+suite: cnpg custom conf test
+templates:
+  - common.yaml
+tests:
+  - it: should pass with custom-conf
+    set:
+      cnpg:
+        main:
+          enabled: true
+          primary: true
+          user: app
+          database: app
+          postgresql:
+            key1: value1
+            key2: '{{ printf "test-tpl" }}'
+    asserts:
+      - documentIndex: &clusterDoc 0
+        isKind:
+          of: Cluster
+      - documentIndex: *clusterDoc
+        isAPIVersion:
+          of: postgresql.cnpg.io/v1
+      - documentIndex: *clusterDoc
+        isSubset:
+          path: spec
+          content:
+            postgresql:
+              key1: value1
+              key2: test-tpl

library/common/Chart.yaml

@@ -15,4 +15,4 @@ maintainers:
 name: common
 sources: null
 type: library
-version: 12.2.15
+version: 12.2.16

library/common/templates/class/_cnpgCluster.tpl

@@ -66,4 +66,7 @@ spec:
     inProgress: false
     reusePVC: on
 
+  postgresql:
+    {{- tpl ( $values.postgresql | toYaml ) $ | nindent 4 }}
+
 {{- end -}}

library/common/values.yaml

@@ -708,6 +708,9 @@ cnpg:
       enablePodMonitor: true
     # -- contains credentials and urls output by generator
     creds: {}
+    # -- contains postgresql settings
+    # ref: https://cloudnative-pg.io/documentation/1.19/postgresql_conf/#the-postgresql-section
+    postgresql: {}
 
 # -- Redis dependency configuration
 # @default -- See below