Remove provider, make spf and dkim optional

[sandipb]

• I removed the provider section because the latest terraform is throwing errors for modules which have their own provider section
• I made the spf/dkim records optional (defaulting to adding them). I had a requirement where I wanted to use this module for fastmail's incredibly useful wildcard domain feature(sending mail to abc@xyz.domain goes to xyz+abc@domain), but I just wanted mx records and not spf/dkim. So I could do:

module "fastmail-mx" {
  source = "github.com/sandipb/terraform-cloudflare-fastmail-mx.git?ref=remove_provider"
  for_each = {
    root     = { sub_domain = "@", dkim_enable = true, spf_enable = true },
    wildcard = { sub_domain = "*", dkim_enable = false, spf_enable = false }
  }
  cf_zone_id   = data.cloudflare_zone.sandipb_dev.id
  cf_api_token = var.cf_api_token
  domain_name  = data.cloudflare_zone.sandipb_dev.name
  sub_domain   = each.value.sub_domain
  dkim_enable   = each.value.dkim_enable
  spf_enable   = each.value.spf_enable
}