add check for correct tier configuration

[RinkiyaKeDad]

Adding some checks for the supported tier configuration could be helpful for users. For example the "dev" performance_tier is only supported on 3GB or lower so this should fail:

resource "dfcloud_datastore" "cache" {
  name = "frontend-cache"

  location = {
    region   = "us-east-1"
    provider = "aws"
  }
 
  tier = {
    max_memory_bytes = 30000000000
    performance_tier = "enhanced"
    replicas         = 1
  }
}

But the error message shown when applying this isn't very clear on what the use might be doing wrong:

│ Error: Error Creating Datastore
│ 
│   with dfcloud_datastore.cache,
│   on main.tf line 13, in resource "dfcloud_datastore" "cache":
│   13: resource "dfcloud_datastore" "cache" {
│ 
│ bad status: 403: datastore configuration not permitted

Similarly certiain sizes like 30Gb aren't supported. We could check for those as well.