Allow API docs for non-Starlark-defined attributes to be (optionally)…

… extracted

Since ExtractorContext is now non-trivial, convert it to a record with an
autobuilder.

This also means finally adding support for tristate, distributions, label dict,
and the various genquery-specific attribute types. (To support extracting the
default value of distributions, we also add support for non-Starlark Set values
to Attribute.valueToStarlark().)

PiperOrigin-RevId: 675169119

third_party/bazel/src/main/protobuf/stardoc_output.proto

@@ -70,6 +70,7 @@ enum AttributeType {
   STRING_LIST_DICT = 11;
   OUTPUT = 12;
   OUTPUT_LIST = 13;
+  LABEL_DICT_UNARY = 14;
 }
 
 // Representation of a Starlark rule definition.