Allow modifying project config values on save

Upon saving the changes to a project config, allow modification of the
values that were entered before the result is committed.

This is useful for plugins which need to validate or change the entered
configuration values before they are saved. For example, a plugin may
require the value to be saved as encrypted text in the configuration
file. This feature would allow the user to input their value in plain
text, and the plugin may override this method to take care of the
encryption. Once the changes are saved, the updated value(s) are shown in
the project config field(s).

Change-Id: I54301b132f421665531ff41643b18933fdbb127c

gerrit-server/src/main/java/com/google/gerrit/server/project/PutConfig.java

@@ -240,6 +240,8 @@ public class PutConfig implements RestModifyView<ProjectResource, Input> {
             if (!value.equals(oldValue)) {
               validateProjectConfigEntryIsEditable(projectConfigEntry,
                   projectState, e.getKey(), pluginName);
+              v.setValue(projectConfigEntry.preUpdate(v.getValue()));
+              value = v.getValue().value;
               try {
                 switch (projectConfigEntry.getType()) {
                   case BOOLEAN: