Add support for role assumption

[wata727]

Fixes #422

As with other provider attributes, evaluate the assume_role block and make it available in TFLint. This PR makes such as the following role assumption works:

provider "aws" {
  assume_role {
    role_arn     = "arn:aws:iam::ACCOUNT_ID:role/ROLE_NAME"
    session_name = "SESSION_NAME"
    external_id  = "EXTERNAL_ID"
  }
}