Add UploadValidationListener extension point.

Add pre-upload extension point to allow plugins to validate upload
operations right before Gerrit begins to send a pack back to the git
client.

A plugin can interrupt the upload by throwing an exception which will
cause the upload to fail and the exception’s message text will be
reported to the git client.

For example, a plugin may want to deny fetches for a specific project.

Change-Id: I5d0aeac51b83bf14150fa3036f71965cf2051243

Documentation/config-validation.txt

diff --git a/Documentation/config-validation.txt b/Documentation/config-validation.txt
index b7843a7..5d23c79 100644
--- a/Documentation/config-validation.txt
+++ b/Documentation/config-validation.txt
@@ -32,6 +32,19 @@
 If the commit fails the validation, the plugin can throw an exception
 which will cause the merge to fail.
 
+[[pre-upload-validation]]
+== Pre-upload validation
+
+
+Plugins implementing the `UploadValidationListener` interface can
+perform additional validation checks before any upload operations
+(clone, fetch, pull). The validation is executed right before Gerrit
+begins to send a pack back to the git client.
+
+If upload fails the validation, the plugin can throw an exception
+which will cause the upload to fail and the exception's message text
+will be reported to the git client.
+
 [[new-project-validation]]
 == New project validation