appmesh-grafana allow to set grafana.ini.server.root_url

**Is your feature request related to a problem?**
I am not able to access grafana via ALB sub path url.

**Is your feature request related to a specific Helm chart, if yes mention name of the chart?**
appmesh-grafana

**Describe the solution you'd like**
Please allow setting grafana.ini key-value pairs. Allow passing these values when install or upgrade. values.yaml needs to provide a placeholder and also config map template should read values.
https://github.com/helm/charts/blob/master/stable/grafana/templates/configmap.yaml

**Describe alternatives you've considered**
I am using a separarte ALB for grafana.

Start with the appmesh-grafana chart's values.yaml and config map template, using the linked Grafana chart configmap.yaml as a reference. Add a configurable set of grafana.ini key-value pairs, including server.root_url, and confirm the values are rendered when installing or upgrading the chart.

Written by the indexing model from the issue text.