Add support for Kafka Fleet output types.

[tobio]

Fixes #395

This includes a breaking change to the current provider schema. Existing outputs defining SSL attributes will require an update as part of upgrading. SSL has been moved from a block to an attribute, so ssl {} becomes ssl = {}.

For example:

resource "elasticstack_fleet_output" "test_output" {
  name = "Test Output"
  type = "elasticsearch"
  config_yaml = yamlencode({
    "ssl.verification_mode" : "none"
  })
  default_integrations = false
  default_monitoring   = false
  hosts = [
    "https://elasticsearch:9200"
  ] 
  ssl {
    certificate_authorities = [
      file("${path.module}/ca.crt")
    ]
    certificate = file("${path.module}/client.crt")
    key         = file("${path.module}/client.key")
  } 
}

becomes:

resource "elasticstack_fleet_output" "test_output" {
  name = "Test Output"
  type = "elasticsearch"
  config_yaml = yamlencode({
    "ssl.verification_mode" : "none"
  })
  default_integrations = false
  default_monitoring   = false
  hosts = [
    "https://elasticsearch:9200"
  ] 
  ssl = {
    certificate_authorities = [
      file("${path.module}/ca.crt")
    ]
    certificate = file("${path.module}/client.crt")
    key         = file("${path.module}/client.key")
  } 
}

[nick-benoit]

Just a few clarifying questions. Looks good! 🚀