Add plugin which allows updating `appVersion` in helm chart according to root project version

**Is your feature request related to a problem? Please describe.**

Given I have a monorepo with project containing application code and Helm Chart, I want the release update not only the `version` but also `appVersion` in the Helm Chart.

**Describe the solution you'd like**

A new plugin `helm-appversion` that by default takes the same value that is in the `"."` package (or other package referenced in the plugin configuration) from `.release-please-manifest.json` and with puts it in the `appVersion` field:

```diff
diff --git a/.release-please-manifest.json b/.release-please-manifest.json
index 6aefea0..c91f231 100644
--- a/.release-please-manifest.json
+++ b/.release-please-manifest.json
@@ -1,4 +1,4 @@
{
- ".": "0.5.0",
- "helm/my-app": "0.2.0"
+ ".": "0.6.0",
+ "helm/my-app": "0.3.0"
}
diff --git a/helm/my-app/Chart.yaml b/helm/my-app/Chart.yaml
index d515421..eb3ef13 100644
--- a/helm/my-app/Chart.yaml
+++ b/helm/my-app/Chart.yaml
@@ -2,5 +2,5 @@ apiVersion: v2
name: MyApp
description: A Helm chart for Kubernetes
type: application
-version: 0.2.0
-appVersion: 0.5.0
+version: 0.3.0
+appVersion: 0.6.0
```

**Describe alternatives you've considered**
N/A

**Additional context**
N/A