Import async-trait crate.

Bug: 164106399
Bug: 158290206
Test: mm
Change-Id: I0c4a491b785134e4ac1d5572e5ff94b8699dc0bc

Cargo.toml.orig

diff --git a/Cargo.toml.orig b/Cargo.toml.orig
new file mode 100644
index 0000000..2921a5f
--- /dev/null
+++ b/Cargo.toml.orig
@@ -0,0 +1,28 @@
+[package]
+name = "async-trait"
+version = "0.1.36"
+authors = ["David Tolnay <dtolnay@gmail.com>"]
+edition = "2018"
+license = "MIT OR Apache-2.0"
+description = "Type erasure for async trait methods"
+repository = "https://github.com/dtolnay/async-trait"
+documentation = "https://docs.rs/async-trait"
+readme = "README.md"
+
+[lib]
+proc-macro = true
+
+[dependencies]
+proc-macro2 = "1.0"
+quote = "1.0"
+syn = { version = "1.0", features = ["full", "visit-mut"] }
+
+[dev-dependencies]
+rustversion = "1.0"
+tracing = "0.1.14"
+tracing-attributes = "0.1.8"
+tracing-futures = "0.2"
+trybuild = { version = "1.0.19", features = ["diff"] }
+
+[package.metadata.docs.rs]
+targets = ["x86_64-unknown-linux-gnu"]