Update on "[executorch][serialization] Rename scalar_type.fbs --> common.fbs"

Allow us to add other shared flatbuffer types into `common.fbs`.

- Rename scalar_type.fbs --> common.fbs
- Rename scalar_type.py --> common_schema.py (differentiate from exir/common.py)

Differential Revision: [D65442256](https://our.internmc.facebook.com/intern/diff/D65442256/)

[ghstack-poisoned]

Author: lucylq

devtools/bundled_program/schema/test/test_schema.py

@@ -14,7 +14,7 @@
 class TestSchema(unittest.TestCase):
     def test_schema_sync(self) -> None:
         # make the test work in both internal and oss.
-        prefix = ("executorch/" if os.path.exists("executorch/schema/common.fbs") else "")
+        prefix = "executorch/" if os.path.exists("executorch/schema/common.fbs") else ""
 
         self.assertTrue(
             filecmp.cmp(