Record: Add warning if TTL has changed

[danaelhe]

Per @DanRStevens suggestion in #1012 , throws a warning if a record's TTL has changed to comply to rfc2181 which states that multiple records for the same fqdn are expected to have the same TTL.

For example, a plan with the following:

resource "digitalocean_domain" "example_com" {
  name = "example.com"
}

resource "digitalocean_record" "test_txt_2" {
  domain   = digitalocean_domain.example_com.name
  type     = "TXT"
  name     = "@"
  value    = "2"
  ttl      = 1800
}

resource "digitalocean_record" "test_txt_1" {
  domain   = digitalocean_domain.example_com.name
  type     = "TXT"
  name     = "@"
  value    = "1"
  ttl      = 60
}

Will result in digitalocean_record.test_txt_1 ttl to change from 60 to 1800. When this occurs, the following warning will be thrown:

│ Warning: digitalocean_record TTL changed
│
│   with digitalocean_record.test_txt_2,
│   on main.tf line 14, in resource "digitalocean_record" "test_txt_2":
│   14: resource "digitalocean_record" "test_txt_2" {
│
│ TTL for Record ID 1739652564 changed from 1800 to 60. This may be due to multiple records with the same fqdn are required to have the same TTL. For reference, see rfc2181,
│ section 5.2: https://www.rfc-editor.org/rfc/rfc2181#section-5.
╵

Apply complete! Resources: 2 added, 0 changed, 0 destroyed.

[andrewsomething]

👍 LGTM!

I did add a suggestion inline that I think makes the warning a bit more clear by being explicit that change was made by DigitalOcean.