Update tonic requirement from 0.3.1 to 0.4.0 in /languages/rust

Updates the requirements on [tonic](https://github.com/hyperium/tonic) to permit the latest version.
- [Release notes](https://github.com/hyperium/tonic/releases)
- [Changelog](https://github.com/hyperium/tonic/blob/master/CHANGELOG.md)
- [Commits](hyperium/tonic@v0.3.1...v0.4.0)

Signed-off-by: dependabot[bot] <support@github.com>

languages/rust/Cargo.toml

@@ -30,7 +30,7 @@ server = []
 
 [dependencies]
 prost = "0.6.1"
-tonic = "0.3.1"
+tonic = "0.4.0"
 
 [dev-dependencies]
 tokio = { version = "0.3.0", features = ["full"] }