Add helm values schema plugin (#728)

**What problem does this PR solve?**:
- Adds make target to update the helm values JSON schema using a helm
plugin.
- Configures pre-commit to check schema.
- Installs helm-schema using nix.

```console
> make pre-commit
▶ running pre-commit
schema-chart.............................................................Failed
- hook id: schema-chart
- files were modified by this hook

make[1]: Entering directory '/home/dlipovetsky/nutanix/capi-runtime-extensions'
JSON schema successfully generated
make[1]: Leaving directory '/home/dlipovetsky/nutanix/capi-runtime-extensions'
```

**Which issue(s) this PR fixes**:
Fixes #

**How Has This Been Tested?**:
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---------

Co-authored-by: Jimmi Dyson <[email protected]>

Author: dlipovetsky

.pre-commit-config.yaml

@@ -4,6 +4,11 @@
 repos:
 - repo: local
   hooks:
+  - id: schema-chart
+    name: schema-chart
+    entry: make schema-chart
+    language: system
+    files: "^charts/cluster-api-runtime-extensions-nutanix/values.yaml$"
   - id: golangci-lint
     name: golangci-lint
     entry: make go-generate lint