move folly optional.pxd from thrift/lib/py3 to folly/python

Summary: It doesn't have any thrift specific code. Moving it to folly allows other folly Cython code use cFollyOptional.

Reviewed By: fried

Differential Revision: D16388752

fbshipit-source-id: 459c564571ba01fb8f22996227120ba631ffc05c

folly/python/optional.pxd

+# Copyright 2019-present Facebook, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+cdef extern from "folly/Optional.h" namespace "folly" nogil:
+    cdef cppclass cOptional "folly::Optional"[T]:
+        cOptional()
+        cOptional(T val)
+        bint has_value()
+        T value()
+        T* get_pointer()
+        void reset()
+        void assign(T&& newValue)