Allow passing existing subnet-group

Is it possible to allow existing subnet-group to be passed? I think its not much of change just some more check.
change in resource "aws_docdb_cluster" "default" {:
`db_subnet_group_name = join("", aws_docdb_subnet_group.default.*.name)`
to
`db_subnet_group_name = var.db_subnet_group ? va.db_subnet_group: join("", aws_docdb_subnet_group.default.*.name)`

and
change in resource "aws_docdb_subnet_group" "default" {
`count = var.enabled ? 1 : 0`
to
`count = var.enabled && !var.db_subnet_group? 1 : 0`

I would make a pull request but I still have no idea how. So just suggestions.

Start at the aws_docdb_cluster and aws_docdb_subnet_group resource definitions mentioned in the issue, then inspect the module variables that control them. Done means an existing subnet group can be passed without creating the default group, while the current default behavior remains unchanged; check any available module validation or examples afterward.

Written by the indexing model from the issue text.