Add config for Bazel CI

[Yannic]

This will ensure gRPC-Web is compatible with the latest Bazel.
If we enable presubmit testing, we can remove Bazel from Kokoro
and speed-up CI.

[stanley-cheung]

@Yannic Thanks for contributing to anything Bazel related :)

So what will this do? Next time when a PR is created, it will go out and find all the bazel targets under this repo and run bazel build and bazel run? Or you mean, whenever Bazel is having a PR, our tests are triggered, (so that they don't break us)?

[Yannic]

Once this is merged and the Bazel CI folks created the pipeline, this will build and test all Bazel targets 1. once a day against the latest Bazel release, and 2. periodically test against Bazel@HEAD to check for compatibility with upcoming breaking changes (e.g. bazelbuild/bazel#8922, which I'll send a PR for soon-ish).
It's also possible to run Bazel CI for every gRPC-Web PR, but that requires to add a webhook to Github so the CI knows about the PR :). You can comment on bazelbuild/continuous-integration#858 and they'll walk you through that process.