Replace 'oneof' with 'optional'

[alexjpwalker]

Problem to Solve

We currently use oneof to denote an optional field. As of Protobuf 3.13+ this is no longer the standard, and is ugly.

Proposed Solution

Replace 'oneof' with 'optional' when all dependants are on Protobuf 3.13+

[flyingsilverfin]

We should use Protobuf's now-standard optional fields in the protocol when we move to TypeDB 3.0