Templating the role statements

Hi there,

is it possible to use a template (eg. involving the username) when creating a dynamic ES role? We used to have user-private index namespace under `user_{username}.*` and would like to switch to a dynamic role for hardened security.

I saw this implemented in other database plugins, eg. MySQL: https://github.com/hashicorp/vault/blob/d9ea865a84998f19dff41812e4c11ad238538f13/sdk/database/helper/dbutil/dbutil.go#L22

Thanks!