Proto filtering: add core MessageFilter and FilterUtil

This CL adds:
- The core MessageFilter class, which will take care
  of filtering on-device.
- The FilterUtil class and related tools/proto_filter
  to generate / test filters offline on the host.

Design doc: go/trace-filtering
Bug: 181306741
Test: see new test and fuzzers introduced

Change-Id: I516669d3b6eee746ab306b1f0edd23e89bfbafb3

gn/BUILD.gn

diff --git a/gn/BUILD.gn b/gn/BUILD.gn
index a297a84..1f799ad 100644
--- a/gn/BUILD.gn
+++ b/gn/BUILD.gn
@@ -195,7 +195,9 @@
 protobuf_full_deps_allowlist = [
   "../src/ipc/protoc_plugin:*",
   "../src/protozero/protoc_plugin:*",
+  "../src/protozero/filtering:filter_util",
   "../src/trace_processor:trace_processor_shell",
+  "../src/protozero/filtering:filter_util",
   "../tools/*",
 ]