protobuf Oneof field serialization+deseriablization not supported by Cadence

[jiaoyli]

Describe the bug
passing OneOf field in request will fail with error:

'''json: cannot unmarshal object into Go struct field'''

To Reproduce
Is the issue reproducible?

• Yes

Steps to reproduce the behavior:
Create a Cadence workflow that takes in a request field that contains a OneOf protobuf field.
Deserializing the input will fail with error '''json: cannot unmarshal object into Go struct field'''

Expected behavior
the oneof field should be supported.

Additional context
The root cause could be because cadence uses encode/json for serialization+deserialization. And this lib doesn't work for oneof fields. My current workaround is: before calling cadence, convert fields that contain Oneof to json string with package: "github.com/gogo/protobuf/jsonpb". then convert it back to proto field in Cadence workflow.