Add ParseFromString for compatibility

[Laurens-T]

I currently use the standard protobuf method ParseFromString quite a bit, but noticed that the generated betterproto dataclasses don't have this method, but they do have SerializeToString for compatibility.

Is there a reason ParseFromString isn't added?

I currently have a reasonable amount of repositories that use ParseFromString, which would require me to refactor quite a bit extra at once without this method.

For clarity, here's an example of what some of the code looks like right now:

syntax = "proto3";

package hello;

message Hello {
  string message = 1;
}

from hello_pb2 import Hello

hello = Hello()
hello.ParseFromString(bytes(Hello))  # dummy example

but it would be nice if I could do the following with betterproto:

from dataclasses import dataclass
import betterproto

@dataclass
class Hello(betterproto.Message):
    """Greeting represents a message you can tell a user."""

    message: str = betterproto.string_field(1)

hello = Hello(message='betterproto')
hello.ParseFromString(bytes(Hello))  # dummy example

[Gobot1234]

There isn't any reason I know of

[simon-saliba]

This is a problem I've encountered before and I believe that the compatibility with the standard protobuf implementation has merit to being supported. I opened a PR for this.