tools: api_booster tool for upgrading Envoy APIs (tool only) (#9329)

This is a beachhead PR for a Clang Libtooling based workflow that automagically updates Envoy's
source tree to the latest API version for every referenced package. So far, ths tool is only capable
of inferring types and performing header fixups, later PRs will expand this.

Risk level: Low
Testing: Manual cleanup of all headers in source/ test/ and include/, all tests pass.

Part of #8082

Signed-off-by: Harvey Tuch <htuch@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ b54d1c366744314dda38aed5f14e7e93323acb41

docs/BUILD

@@ -10,6 +10,7 @@ package_group(
 # This is where you add protos that will participate in docs RST generation.
 proto_library(
     name = "protos",
+    visibility = ["//visibility:public"],
     deps = [
         "//envoy/admin/v2alpha:pkg",
         "//envoy/api/v2:pkg",