AWS-for-fluent-bit should allow custom configuration for elasticsearch parameters.

**Is your feature request related to a problem?**
I am attempting to configure the aws-for-fluent-bit chart to work with AWS ElasticSearch, but due to limited customizability in the elasticsearch section, am unable to do so in an automated fashion. I must manually edit the configmap post-install.

**Is your feature request related to a specific Helm chart, if yes mention name of the chart?**
aws-for-fluent-bit

**Describe the solution you'd like**
Add an "extra-values" section and append that to the elasticsearch section in the configmap.

**Describe alternatives you've considered**
Either fork the chart, or edit the configmap manually once the chart is installed.

Start in the aws-for-fluent-bit Helm chart and inspect how the Elasticsearch section is rendered into its ConfigMap. Add the requested extra-values configuration and verify that supplied values are appended to the Elasticsearch section without changing the default output.

Written by the indexing model from the issue text.