Parameterize access policies json for more flexibility

[bmbferreira]

what

Adds a new parameter to be able to pass a json string with a custom access policy to set for the elasticsearch.

why

In my opinion, the access policies are too "opinionated" on this module, especially when it is on "vpc mode". I think it should be more flexible and allow to customize it however we want and not having it based on the iam_role_arn variable.

references

N/A