Add a function to create config_setting(s)

The use case for this is creating config_setting(s) specific to an apex
variant and selecting stub/impl in that config_setting. We likely need
only a handful of such config_setting(s), but determining that list
requires iterating the build graph.

Test: go test ./bp2build
Change-Id: I9aa552e3d0bcf67513023c3a7d4bbf8fae464ee4
diff --git a/bp2build/bzl_conversion_test.go b/bp2build/bzl_conversion_test.go
index a8e557d..fa1bf8a 100644
--- a/bp2build/bzl_conversion_test.go
+++ b/bp2build/bzl_conversion_test.go
@@ -108,6 +108,7 @@
         "string_literal_prop": attr.string(),
         "string_prop": attr.string(),
         "string_ptr_prop": attr.string(),
+        "test_config_setting": attr.bool(),
     },
 )
 
@@ -139,6 +140,7 @@
         "string_literal_prop": attr.string(),
         "string_prop": attr.string(),
         "string_ptr_prop": attr.string(),
+        "test_config_setting": attr.bool(),
     },
 )
 
@@ -170,6 +172,7 @@
         "string_literal_prop": attr.string(),
         "string_prop": attr.string(),
         "string_ptr_prop": attr.string(),
+        "test_config_setting": attr.bool(),
         # test_prop start
 #         "test_string_prop": attr.string(),
         # test_prop end