feat: added ParseFromString method to message for compatibility

[simon-saliba]

This PR adds the ParseFromString attribute to the message class for compatibility with the official Google protobuf implementation and method exposure.
Closes #323

[Laurens-T]

Thanks!

[Gobot1234]

Suggested change

	def ParseFromString(self, data: bytes) -> T:
	def ParseFromString(self: T, data: bytes) -> T:

[Gobot1234]

Isn't this slightly different from the offical implementation which clears any currently set values? https://github.com/protocolbuffers/protobuf/blob/2a2a9b6e64e0cda49b0e5377cad58c790087e1c9/python/google/protobuf/message.py#L193-L199

[Laurens-T]

This indeed seems to be the case.

I've added a simple example which shows the difference in result from ParseFromString with the one from this PR:

syntax = "proto3";

package some.package.name;

message Cat {
  int64 age = 1;
  int64 lives = 2;
}

def ParseFromString(self, data: bytes):
    return self.parse(data)

if __name__ == '__main__':
    # Create bytes
    cat = Cat(
        age=5,
        # lives=9,
    )

    data = cat.SerializeToString()

    # Dynamically add ParseFromString to the Cat class
    Cat.ParseFromString = ParseFromString

    # ParseFromString for the betterproto cat and the pb2 cat
    cat_betterproto = Cat(age=3, lives=8)
    cat_pb2 = Cat_pb2(age=3, lives=8)

    cat_betterproto.ParseFromString(data)
    cat_pb2.ParseFromString(data)

    print(cat_betterproto)  # Cat(age=5, lives=8), lives are not cleared
    print(cat_pb2)  # age: 5, lives are cleared
    assert cat_pb2.lives == 0