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.

首先阅读 DiscoveryTypeVertex::GetFieldNumber，以及生成的 proto 文件与 discovery document 之间的比较。检查带有和不带有 optional 关键字的 proto3 字段是如何处理的，并将该行为与 has_presence 进行比较。当等效字段在适当情况下始终复用现有字段编号时，即表示完成。

由索引模型根据 Issue 内容生成。