Reconsider whether the lack/addition of an explicit optional keyword indicates a change in field type

When comparing the current generation of a proto file to the discovery document being parsed, in `DiscoveryTypeVertex::GetFieldNumber` we interrogate the existence of the `optional` keyword in the .proto file as a criteria as to whether or not we can reuse the field number. As these proto files use proto3 syntax, this check may be superfluous and fields with and without the `optional` keyword may be equivalent. This issue was motivated by protobuf deprecating `has_optional_keyword` in favor of `has_presence`. `has_presence` treats the fields as equivalent.

Start by reading DiscoveryTypeVertex::GetFieldNumber and the comparison between the generated proto file and the discovery document. Check how proto3 fields with and without the optional keyword are treated, and compare that behavior with has_presence. Done means equivalent fields consistently reuse the existing field number when appropriate.

Written by the indexing model from the issue text.