api: add bootstrap extensions (#11413)

Commit Message:
Adds a configurable server wide extension hook point. Allow extensions to instantiate singletons / context managers with configs in bootstrap.

Additional Description:
Risk Level: Low (not used API)
Testing: unittest, mock
Docs Changes: protodoc
Release Notes: N/A as no real extension lives today.
Fixes #11219
Signed-off-by: Lizan Zhou <lizan@tetrate.io>

Mirrored from https://github.com/envoyproxy/envoy @ 8cb2958f29af3e45e87653356b6099bd3a4cbfbf

envoy/config/bootstrap/v3/bootstrap.proto

@@ -7,6 +7,7 @@ import "envoy/config/core/v3/address.proto";
 import "envoy/config/core/v3/base.proto";
 import "envoy/config/core/v3/config_source.proto";
 import "envoy/config/core/v3/event_service_config.proto";
+import "envoy/config/core/v3/extension.proto";
 import "envoy/config/core/v3/socket_option.proto";
 import "envoy/config/listener/v3/listener.proto";
 import "envoy/config/metrics/v3/stats.proto";
@@ -35,7 +36,7 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 // <config_overview_bootstrap>` for more detail.
 
 // Bootstrap :ref:`configuration overview <config_overview_bootstrap>`.
-// [#next-free-field: 21]
+// [#next-free-field: 22]
 message Bootstrap {
   option (udpa.annotations.versioning).previous_message_type =
       "envoy.config.bootstrap.v2.Bootstrap";
@@ -181,6 +182,10 @@ message Bootstrap {
   // :ref:`use_tcp_for_dns_lookups <envoy_api_field_config.cluster.v3.Cluster.use_tcp_for_dns_lookups>` are
   // specified.
   bool use_tcp_for_dns_lookups = 20;
+
+  // Specifies optional bootstrap extensions to be instantiated at startup time.
+  // Each item contains extension specific configuration.
+  repeated core.v3.TypedExtensionConfig bootstrap_extensions = 21;
 }
 
 // Administration interface :ref:`operations documentation

envoy/config/bootstrap/v4alpha/bootstrap.proto

@@ -7,6 +7,7 @@ import "envoy/config/core/v4alpha/address.proto";
 import "envoy/config/core/v4alpha/base.proto";
 import "envoy/config/core/v4alpha/config_source.proto";
 import "envoy/config/core/v4alpha/event_service_config.proto";
+import "envoy/config/core/v4alpha/extension.proto";
 import "envoy/config/core/v4alpha/socket_option.proto";
 import "envoy/config/listener/v4alpha/listener.proto";
 import "envoy/config/metrics/v4alpha/stats.proto";
@@ -34,7 +35,7 @@ option (udpa.annotations.file_status).package_version_status = NEXT_MAJOR_VERSIO
 // <config_overview_bootstrap>` for more detail.
 
 // Bootstrap :ref:`configuration overview <config_overview_bootstrap>`.
-// [#next-free-field: 21]
+// [#next-free-field: 22]
 message Bootstrap {
   option (udpa.annotations.versioning).previous_message_type =
       "envoy.config.bootstrap.v3.Bootstrap";
@@ -173,6 +174,10 @@ message Bootstrap {
   // :ref:`use_tcp_for_dns_lookups <envoy_api_field_config.cluster.v4alpha.Cluster.use_tcp_for_dns_lookups>` are
   // specified.
   bool use_tcp_for_dns_lookups = 20;
+
+  // Specifies optional bootstrap extensions to be instantiated at startup time.
+  // Each item contains extension specific configuration.
+  repeated core.v4alpha.TypedExtensionConfig bootstrap_extensions = 21;
 }
 
 // Administration interface :ref:`operations documentation