Kafka connection: add support for AWS IAM authentication for Kafka

[bobbyiliev]

As per the PR here https://github.com/MaterializeInc/materialize/pull/28683 we need to also incorporate this into the Terraform provider.

So far the SQL syntax for this is:

CREATE CONNECTION kafka_msk TO KAFKA (
    BROKER 'msk.mycorp.com:9092',
    SECURITY PROTOCOL = 'SASL_SSL',
    AWS CONNECTION = kafka_msk
);

If the above remains as final, it could be translated to:

resource "materialize_connection_kafka" "kafka_conn_ssh_default" {
  name = "kafka_conn_ssh_default"
  kafka_broker {
    broker = "redpanda:9092"
  }
  // additional settings
  aws_connection {
    name          = "kafka_msk"
    database_name = "materialize"
    schema_name   = "public"
  }