Add ParseFromString for compatibility

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:
```protobuf
syntax = "proto3";

package hello;

message Hello {
string message = 1;
}
```

```python
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:
```python
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
```

Start by reading the betterproto Message API and the existing SerializeToString implementation to find where compatibility methods belong. Use the Hello example in the issue to check how parsing should be exposed and verify that the resulting dataclass matches the standard protobuf behavior.

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