tonic-build: option to emit Arc<Self> as server receiver (#1352)

This implements an option to emit `Arc<Self>` instead of `&self` in
server traits. This enables implementor to reference `Self` data for
indefinite duration, which may be useful for streaming requests.

Fixes: #1351

tests/use_arc_self/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+authors = ["Aurimas Blažulionis <[email protected]>"]
+edition = "2021"
+license = "MIT"
+name = "use_arc_self"
+publish = false
+version = "0.1.0"
+
+[dependencies]
+futures = "0.3"
+prost = "0.11"
+tonic = {path = "../../tonic", features = ["gzip"]}
+
+[build-dependencies]
+tonic-build = {path = "../../tonic-build" }
+
+[package.metadata.cargo-machete]
+ignored = ["prost"]