feature: Adds option for single account id config

[mikef-nl]

#11

[shoekstra]

Looks good, thanks!

If you can please fix variable formatting then this is good to go 🚀

[shoekstra]

Suggested change

	validation {
	condition = var.config.zone_name != "" || var.config.single_account_id != ""
	error_message = "Either zone_name or single_account_id is required"
	}
	validation {
	condition = var.config.zone_name != "" || var.config.single_account_id != ""
	error_message = "Either zone_name or single_account_id is required"
	}

Please fix the formatting here.