bootstrap: make admin field optional. (#4760)

This is a follow up to #4726. In #4726, the access log path became optional, but the admin field
was not itself marked optional. This then led to server_fuzz_test trivially passing due to an early
PGV validation exception, and ~20 bugs being closed out by oss-fuzz. This PR completes the admin
optionality changes.

Risk Level: Low
Testing: Unit tests updated.

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

Mirrored from https://github.com/envoyproxy/envoy @ 86790c2367558160282d8b0afa1c5e4698e2fed3

envoy/config/bootstrap/v2/bootstrap.proto

@@ -124,7 +124,7 @@ message Bootstrap {
   Runtime runtime = 11;
 
   // Configuration for the local administration HTTP server.
-  Admin admin = 12 [(validate.rules).message.required = true, (gogoproto.nullable) = false];
+  Admin admin = 12;
 
   // Optional overload manager configuration.
   envoy.config.overload.v2alpha.OverloadManager overload_manager = 15;

test/validate/pgv_test.cc

@@ -54,6 +54,7 @@ template <class Proto> struct TestCase {
 // from data plane API.
 int main(int argc, char* argv[]) {
   envoy::config::bootstrap::v2::Bootstrap invalid_bootstrap;
+  invalid_bootstrap.mutable_runtime();
   // This is a baseline test of the validation features we care about. It's
   // probably not worth adding in every filter and field that we want to valid
   // in the API upfront, but as regressions occur, this is the place to add the