feat: get, set methods for OpenEnum

These are meant to be the primary ways to get and set
open enumeration fields, instead of the getters and setters generated
by Message derive.

Author: mzabaluev

prost/src/open_enum.rs

@@ -1,5 +1,5 @@
 use crate::encoding::{DecodeContext, WireType};
-use crate::{DecodeError, Message};
+use crate::{DecodeError, Message, UnknownEnumValue};
 
 use bytes::{Buf, BufMut};
 
@@ -128,6 +128,23 @@ impl<T> OpenEnum<T> {
         }
     }
 
+    /// If the value of the open enum is known, returns it in `Ok`, otherwise
+    /// returns an `Err` with the unknown value.
+    pub fn get(&self) -> Result<T, UnknownEnumValue>
+    where
+        T: Clone,
+    {
+        match self {
+            Self::Known(v) => Ok(v.clone()),
+            Self::Unknown(r) => Err(UnknownEnumValue(*r)),
+        }
+    }
+
+    /// Sets the value of receiver to the provided known value.
+    pub fn set(&mut self, value: T) {
+        *self = Self::Known(value);
+    }
+
     /// If the value of the open enum is known, returns it in `Some`, otherwise
     /// returns `None`.
     pub fn known(self) -> Option<T> {