Add docs for `ExternalProviders` in TestCase

### Use cases
There are some useful utility provider like [`null`](https://registry.terraform.io/providers/hashicorp/null/latest/docs/resources/resource) and [`http`](https://registry.terraform.io/providers/hashicorp/http/latest), which might be used in the acceptance tests of the another provider.

### Attempted solutions
Importing those providers as Go packages and placing them into the provider factory. This doesn't work well because many providers doesn't follow Golang's package standard.

See this example: https://github.com/hashicorp/terraform-provider-http/pull/428

### Proposal
Maybe we can have a new field on `Test` struct to allow setup of other providers?