cmd/protoc-gen-openapi: additional_bindings duplicate operationId

It looks like the changes merged in #367 generate duplicate `operationId`'s.

You can see this in the test example:

* https://github.com/google/gnostic/blob/3751138311725a0e9d353b805b2e121c94155d18/cmd/protoc-gen-openapi/examples/tests/additional_bindings/openapi.yaml#L13
* https://github.com/google/gnostic/blob/3751138311725a0e9d353b805b2e121c94155d18/cmd/protoc-gen-openapi/examples/tests/additional_bindings/openapi.yaml#L37

According to the spec these must be unique:

> Unique string used to identify the operation. The id MUST be unique among all operations described in the API. Tools and libraries MAY use the operationId to uniquely identify an operation, therefore, it is RECOMMENDED to follow common programming naming conventions.

- [Source](https://github.com/OAI/OpenAPI-Specification/blob/3.0.1/versions/3.0.1.md#operationId)

In addition to the auto-generated `operationId` we'll need to figure out how to resolve it when manually applied:

```protobuf
service TestService {
rpc Test(TestRequest) returns (TestResponse) {
option (google.api.http) = {
get: "/test"
additional_bindings {
get: "/test2"
}
};
option (openapi.v3.operation) = {
operation_id: "test"
};
}
}
```