Create data source for external groups

[kfcampbell]

Fixes #1145.

When given a Terraform snippet like the following:

provider "github" {}

terraform {
  required_providers {
    github = {
      source  = "integrations/github"
    }
  }
}

locals {
  local_groups = "${data.github_external_groups.example_external_groups}"
}

output "groups" {
  value = local.local_groups
}

this PR will produce a state that looks like so:

Outputs:

groups = {
    external_groups = [
        {
            group_id   = 0
            group_name = "Test Group 0"
            updated_at = "2021-02-22T19:25:55Z"
        },
        {
            group_id   = 1
            group_name = "Test Group 1"
            updated_at = "2021-02-22T19:25:55Z"
        }
    ]
    id              = "/orgs/TEST_ORG_NAME/external-groups"
}

This will be helpful in retrieving groups for use in the EMU group mapping resource.

[kfcampbell]

@micbegind would the state captured in the PR description fix your issue?

[philthethrill99]

@kfcampbell Yes this would fix our issue