Add plugin-manager as core plugin

Having a mechanism to discover, install and upgrade plugins
is an expected feature for a core distribution.

Plugin manager has been included since the very beginning in
the binary packages (RPMs and Debs) and in the famous Docker
image that has been pulled over 100k times.

The only place where the plugin-manager is not present is
the WAR package: this proposal fills the gap and aligns the
package with the rest of the current distributions.

Feature: Issue 10717
Change-Id: Ia5b9c74f177aac135100d3cf138a38697a2db0ca

.gitignore

@@ -41,6 +41,7 @@
 !/plugins/external_plugin_deps.bzl
 !/plugins/gitiles
 !/plugins/hooks
+!/plugins/plugin-manager
 !/plugins/replication
 !/plugins/reviewnotes
 !/plugins/singleusergroup

.gitmodules

@@ -28,6 +28,11 @@
 	url = ../plugins/hooks
 	branch = .
 
+[submodule "plugins/plugin-manager"]
+	path = plugins/plugin-manager
+	url = ../plugins/plugin-manager
+	branch = .
+
 [submodule "plugins/replication"]
 	path = plugins/replication
 	url = ../plugins/replication

Documentation/config-plugins.txt

@@ -110,6 +110,20 @@ Documentation] |
 link:https://gerrit.googlesource.com/plugins/hooks/+doc/master/src/main/resources/Documentation/config.md[
 Configuration]
 
+[[plugin-manager]]
+=== plugin-manager
+
+This plugins provides an initial wizard to discover and install Gerrit plugins.
+Per default GerritForge CI is used to download the plugin artifacts from, but
+this can be changed per plugin configuration.
+
+link:https://gerrit-review.googlesource.com/admin/repos/plugins/plugin-manager[
+Project]
+link:https://gerrit.googlesource.com/plugins/plugin-manager/+doc/master/src/main/resources/Documentation/about.md[
+Documentation]
+link:https://gerrit.googlesource.com/plugins/plugin-manager/+doc/master/src/main/resources/Documentation/config.md[
+Configuration]
+
 [[replication]]
 === replication
 

plugins/plugin-manager

@@ -0,0 +1 @@
+Subproject commit e1cc65b2fd3b2e2815181e069fcbefb1e74e53bf

tools/bzl/plugins.bzl

@@ -5,6 +5,7 @@ CORE_PLUGINS = [
     "download-commands",
     "gitiles",
     "hooks",
+    "plugin-manager",
     "replication",
     "reviewnotes",
     "singleusergroup",