rust: add tempfile dependency (#4335)

Summary:
The [`tempfile`] crate is the standard Rust library for handling
temporary files and directories. We anticipate using this for tests that
want to set up a directory of event files.

[`tempfile`]: https://crates.io/crates/tempfile

Test Plan:
It builds: `bazel build //third_party/rust:tempfile`.

wchargin-branch: rust-dep-tempfile

Author: wchargin

tensorboard/data/server/Cargo.lock

@@ -36,6 +36,7 @@ tonic = "0.3.1"
 
 [dev-dependencies]
 prost-build = "0.6.1"
+tempfile = "3.1.0"
 tonic-build = "0.3.1"
 
 [[bin]]