Summary:
This includes: `tonic`, a gRPC framework; `tonic-build`, its compiler
for protobuf bindings; `tokio`, an async runtime; and `async-stream`,
`async-trait`, and `futures-core`, helpers for streaming gRPC methods.

Test Plan:
All crates build without any manual configuration:

```
bazel build //third_party/rust:{tonic{,_build},tokio,async_{stream,trait},futures_core}
```

wchargin-branch: rust-dep-tonic-stack
wchargin-source: 0fa8ea005745e7194bde20da6fb1b9871c05822a

Summary:
This patch switches our Rust protocol buffer bindings from `prost-build`
to `tonic-build`. This generates the same protobuf message types, since
Tonic interoperates with Prost, but can now also generate gRPC stubs.

Test Plan:
Check the doc server to see that our existing proto types still look
good, in the `rustboard_core::proto::tensorboard` package. We don’t have
gRPC services to generate yet. A follow-up commit will add a demo server
as proof of concept.

wchargin-branch: rust-compile-grpc
wchargin-source: c5090247a8a25c1902d4b42597ec4d4ae2593779

Summary:
This patch defines a simple “demo” service with one RPC, which adds a
sequence of numbers. It includes a Tonic server for this service to
demonstrate the end-to-end setup.

Test Plan:
In one shell, run `bazel run -c opt //tensorboard/data/server`. In
another shell, use `grpc_cli` to send an RPC to localhost port 6789:

```
grpc_cli --channel_creds_type=insecure \
    --protofiles tensorboard/data/server/demo.proto \
    call localhost:6789 Demo.Add "term: 2 term: 2"
```

This should print a response like `sum: 4`. On my machine, it completes
in 5.2 ± 2.6 ms (methodology: run `hyperfine` on the above command).
This seems reasonably fast given that it has to establish a connection,
whereas a Python gRPC client will keep a connection open. It’s also well
below the 40ms magic number for `TCP_NODELAY` issues.

wchargin-branch: rust-demo-tonic-server
wchargin-source: 25d9fcbf2279742cdffde0c5357711244e9afef2

wchargin-branch: rust-compile-grpc
wchargin-source: 6d888a758d42d318467189245ebcc619acaefe9c

wchargin-branch: rust-demo-tonic-server
wchargin-source: 2181a31c718433d5113cbb1fe2e51c7c98193cc1

wchargin-branch: rust-demo-tonic-server
wchargin-source: 2181a31c718433d5113cbb1fe2e51c7c98193cc1

wchargin-branch: rust-compile-grpc
wchargin-source: ba5e9a59b4b4f95e86b1f7a4a0b0d5c28c446476

wchargin-branch: rust-demo-tonic-server
wchargin-source: 89356565a9557266c069dcfdf0422f2e2225e3ab

# Conflicts:
#	tensorboard/data/server/gen_protos_tool.rs

wchargin-branch: rust-demo-tonic-server
wchargin-source: 89356565a9557266c069dcfdf0422f2e2225e3ab

wchargin-branch: rust-demo-tonic-server
wchargin-source: eb0bce9732ce4810d7152ec6eb47ba67a8d85d5e

# Conflicts:
#	tensorboard/data/server/gen_protos_tool.rs

wchargin-branch: rust-demo-tonic-server
wchargin-source: eb0bce9732ce4810d7152ec6eb47ba67a8d85d5e