[WIP] Bazel Build Event Stream support

[aherrmann]

This PR adds support to connect Buck2 to a Bazel Build Event Stream server as found among some Bazel remote build execution platforms. It repurposes the existing RemoteEventSink used to connect to Scribe within Meta's internal Buck2 version and connects it to a Bazel Build Event Service over gRPC instead, and it adds a translation layer between Buck2 build events and Bazel build events.

This is a work in progress, only very few build events are translated so far. To test this code you can

• build Buck2,
• change into the examples/hello_world directory,
• configure a BES service by setting the BES_URI environment-variable, and
• start a build.

$ cargo build --bin=buck2
$ cd examples/hello_world
$ BES_URI=XYZ ../../target/debug/buck2 build :main

For example, with BuildBuddy, if you run bazel run server -- -app.log_level debug locally, set BES_URI=http://localhost:1985.

The Buck2 Trace-ID is used as the BES invocation ID and you can see a Buck2 build on the UI under e.g. http://localhost:8080/invocation/e3123cfa-c350-432b-9784-8b3fe48bed14.

• BuckEvent: provide a Buck Event Publisher proto
• Add buck2_event_publisher_proto to Cargo.toml
• Add Bazel Build Event Protocol/Stream protobuf
• Implement RemoteEventSink to publish to Bazel BES

[nlopezgi]

Hi @aherrmann , thanks for putting this together, we are really looking forward to it. I tested it out and had a couple of questions:

• I see the TLS bit is marked on a TODO as not enabled yet, are you planning on setting this up as part of this CL or a future one?
• Is there any way that I can get some verbose logs from the service publishing BuildEvents? I did not find a way to see anything from this interaction.

Thanks again!