tonic-build: support boxing fields

This exposes tokio-rs/prost#802 in Tonic,
though obviously until that's merged and released this diff does
nothing.

tonic-build/src/prost.rs

@@ -26,6 +26,7 @@ pub fn configure() -> Builder {
         message_attributes: Vec::new(),
         enum_attributes: Vec::new(),
         type_attributes: Vec::new(),
+        boxed: Vec::new(),
         server_attributes: Attributes::default(),
         client_attributes: Attributes::default(),
         proto_path: "super".to_string(),
@@ -229,6 +230,7 @@ pub struct Builder {
     pub(crate) type_attributes: Vec<(String, String)>,
     pub(crate) message_attributes: Vec<(String, String)>,
     pub(crate) enum_attributes: Vec<(String, String)>,
+    pub(crate) boxed: Vec<String>,
     pub(crate) server_attributes: Attributes,
     pub(crate) client_attributes: Attributes,
     pub(crate) proto_path: String,
@@ -332,6 +334,14 @@ impl Builder {
         self
     }
 
+    /// Add additional boxed fields.
+    ///
+    /// Passed directly to `prost_build::Config.boxed`.
+    pub fn boxed<P: AsRef<str>>(mut self, path: P) -> Self {
+        self.boxed.push(path.as_ref().to_string());
+        self
+    }
+
     /// Add additional attribute to matched server `mod`s. Matches on the package name.
     pub fn server_mod_attribute<P: AsRef<str>, A: AsRef<str>>(
         mut self,
@@ -479,6 +489,9 @@ impl Builder {
         for (prost_path, attr) in self.enum_attributes.iter() {
             config.enum_attribute(prost_path, attr);
         }
+        for prost_path in self.boxed.iter() {
+            config.boxed(prost_path);
+        }
         if self.compile_well_known_types {
             config.compile_well_known_types();
         }