Add data: env and cluster

[rjudin]

Issue: as long as #33 in pending state cluster can't be created in code. That disallows to create of service accounts with API keys for this cluster which required for provider "kafka"

Proposed resolution: Please add data objects that could resolve the issue by fetching existing resources (without importing to tfstate)

ENV

data "confluentcloud_environment" "this" {
  name = "DEV"
}

which return attributes:

{
"id" = "env-xxxxx"
"name" = "DEV"
}

---

CLUSTER

data "confluentcloud_kafka_cluster" "this" {
  environment_id = data.confluentcloud_environment.this.id
  name           = "DEV-BASIC"
}

which return attributes:

{
"bootstrap_servers" = "SASL_SSL://pkc-xyxyx.ap-southeast-1.aws.confluent.cloud:9092"
"id" = "lkc-09034"
}

Thank you!

[wheresalice]

This looks like a duplicate of #28 - albeit with a little more detail