[roll] Roll fuchsia [build][bazel] Group examples into single GN bazel_action() target.

Crossing the GN -> Bazel boundary is costly, even if there is nothing
to do, invoking Bazel can only be done in sequence (from Ninja's
point of view), and results in an analysis phase that can take
1 second per invocation.

To reduce the time spent building the bazel examples, group them
under a single bazel_action() that references a single Bazel target
that depends on all other example targets in the Bazel graph.

This allows Bazel to dispatch build actions, if necessary in parallel,
and if there is nothing to do, a single Bazel invocation, instead of
5, will be performed.

+ Add //build/bazel/rules/stamp_group.bzl to define a new
  "stamp_group()" Bazel rule that can be used to define a
  Bazel target that acts as a filegroup() but also generates
  a stamp file once all its dependencies are updated.

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1019853
Original-Revision: bf3c3951a896204ace932d4a4961af5f72d46c0e
GitOrigin-RevId: 7b9145a9770308f48b89c17654f160cd516089aa
Change-Id: If829d9d388b41916848bb63f2428e55d2a8dd315