[Enhancement] Add a Clear method to the Message class for compatibility.

When converting existing code to use this module over standard protobuf, one issue raised was the lack of `ParseFromString`. This has already been mentioned at #323, with a PR in the works at #336. However, this is blocked because it does not perform the same calls as the standard, namely, `Clear`. In my use of this module, i have worked around this issue by simply creating a new instance (which is always the default) instead of using an existing one, at which point just calling `parse` works fine.

Ideally, we'd need a `clear` function (preferably lowercase to match python syntax, but might want TitleCase for compatibility) on the abstract Message class and an implementation that resets all fields to their default values. We'd need to confirm that our implementation matches standard protobuf when it comes to user-defined default values.

to be clear, this isn't a bug, and it doesn't prevent the use of this module.
I'd look into addressing this myself but the logic used to set default values eludes me, at least at first glance.

从抽象的 Message 类以及现有的 parse 和默认值逻辑开始；将它们的行为与标准 protobuf 进行比较，包括用户定义的默认值。当 clear 方法将所有字段重置为其默认值，并支持 #323 和 #336 中描述的兼容性工作时，即表示完成。

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