chore(reflection): Make prost-types dependency optional (#1610)

Author: tottoto

tonic-reflection/Cargo.toml

@@ -22,12 +22,12 @@ all-features = true
 rustdoc-args = ["--cfg", "docsrs"]
 
 [features]
-server = ["dep:tokio", "dep:tokio-stream"]
+server = ["prost-types", "dep:tokio", "dep:tokio-stream"]
 default = ["server"]
 
 [dependencies]
 prost = "0.12"
-prost-types = "0.12"
+prost-types = {version = "0.12", optional = true}
 tokio = { version = "1.0", features = ["sync", "rt"], optional = true }
 tokio-stream = {version = "0.1", features = ["net"], optional = true }
 tonic = { version = "0.10", path = "../tonic", default-features = false, features = ["codegen", "prost"] }