Support project-level tag key/value contexts in project factory (#3714)

* cross-project tag context

* improve regression test

* add tag contexts to README contexts table

modules/project-factory/variables.tf

@@ -30,6 +30,7 @@ variable "context" {
     project_numbers       = optional(map(string), {})
     pubsub_topics         = optional(map(string), {})
     storage_buckets       = optional(map(string), {})
+    tag_keys              = optional(map(string), {})
     tag_values            = optional(map(string), {})
     vpc_host_projects     = optional(map(string), {})
     vpc_sc_perimeters     = optional(map(string), {})