[Enhancement][OpenSearch Dashboards] Add support for disabling automountServiceAccountToken

[gsmith-sas]

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

We needed to disable the auto-mounting of ServiceAccount API token to the OpenSearch Dashboards pod. Although the OpenSearch Helm chart supports this, the OpenSearch Dashboards Helm chart does not.

Describe the solution you'd like

I will submit a PR shortly that adds support for doing this to the OpenSearch Dashboards Helm chart. In the PR, we have maintained the current default behavior but allow users to explicitly request to disable this automounting (by setting the serviceAccount.automountServiceAccountToken key)

Describe alternatives you've considered

We originally patched the Service Account resource after it was created but addressing this with the Helm chart eliminates the need for such a patch. It also "cleaner" since the resource is created properly initially.

Additional context
Add any other context or screenshots about the feature request here.

[gsmith-sas]

This enhancement was added in #683.