encoding/openapi: add FieldFilter option

This option allows excluding certain fields from the output.

The exclusion happens through a single regexp,
where a field name is qualified by its object type
and name.

Change-Id: I60913d92eec8ad60da73904e016c75fc61dcce98
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2444
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/openapi/testdata/simple-filter.json b/encoding/openapi/testdata/simple-filter.json
new file mode 100644
index 0000000..a971c73
--- /dev/null
+++ b/encoding/openapi/testdata/simple-filter.json
@@ -0,0 +1,37 @@
+{
+   "openapi": "3.0.0",
+   "info": {
+      "title": "test",
+      "version": "v1"
+   },
+   "components": {
+      "schema": {
+         "MyStruct": {
+            "type": "object",
+            "required": [
+               "mediumNum",
+               "smallNum",
+               "float",
+               "double"
+            ],
+            "properties": {
+               "double": {
+                  "type": "number",
+                  "format": "double"
+               },
+               "float": {
+                  "type": "number",
+                  "format": "float"
+               },
+               "mediumNum": {
+                  "type": "integer",
+                  "format": "int32"
+               },
+               "smallNum": {
+                  "type": "integer"
+               }
+            }
+         }
+      }
+   }
+}
\ No newline at end of file