bazel: Integrate bazel build file generator

To generate/update BUILD.bazel files for any particular client or a batch of clients:
```
bazel run //:build_gen -- --src=google/example/library
```

PiperOrigin-RevId: 286958627

BUILD.bazel

@@ -1,10 +1,6 @@
 load("@bazel_tools//tools/build_defs/pkg:pkg.bzl", "pkg_tar")
 
-# Target used by Cloud Build to build GAPIC libraries.
-# To find all available rules, use:
-#   $ grep -Er '\-(java|go|php)"'
-pkg_tar(
-    name = "gapic-cloud-build",
-    deps = [
-    ],
+alias(
+    name = "build_gen",
+    actual = "@com_google_api_codegen//rules_gapic/bazel:build_file_generator",
 )

WORKSPACE

@@ -61,8 +61,8 @@ rules_proto_toolchains()
 # section
 http_archive(
     name = "com_google_api_codegen",
-    strip_prefix = "gapic-generator-e4c0fcaf2b2e2016a468b341283a260581c2ee74",
-    urls = ["https://github.com/googleapis/gapic-generator/archive/e4c0fcaf2b2e2016a468b341283a260581c2ee74.zip"],
+    strip_prefix = "gapic-generator-08cef92e891da35cfc49a916c302d8817c88e1f2",
+    urls = ["https://github.com/googleapis/gapic-generator/archive/08cef92e891da35cfc49a916c302d8817c88e1f2.zip"],
 )
 
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