tendermint-proto: gate tonic import on grpc feature

Co-authored-by: Tomas Tauber <[email protected]>
Co-authored-by: Tony Arcieri <[email protected]>

proto/Cargo.toml

@@ -13,6 +13,12 @@ description = """
     tendermint-proto is a the Rust implementation of the Tendermint proto structs.
     """
 
+[features]
+default = []
+grpc = ["grpc-server", "grpc-client"]
+grpc-server = ["tonic"]
+grpc-client = ["tonic"]
+
 [package.metadata.docs.rs]
 all-features = true
 
@@ -27,6 +33,7 @@ num-traits = { version = "0.2", default-features = false }
 num-derive = { version = "0.3", default-features = false }
 time = { version = "0.3", default-features = false, features = ["macros", "parsing"] }
 flex-error = { version = "0.4.4", default-features = false }
+tonic = { version = "0.9", optional = true }
 
 [dev-dependencies]
 serde_json = { version = "1.0", default-features = false, features = ["alloc"] }