Add the context-padding parameter to the comment context

This parameter allows padding the context lines with extra lines before
and after the comment range. This parameter works only if the
enable-context parameter is set to true.

This change is backward compatible: if the new parameter is not set or
set to zero, only the lines of the comment range are returned.

Change-Id: I2d44fb852c68ba484cbf94adff912b7ee522f9c5

proto/cache.proto

diff --git a/proto/cache.proto b/proto/cache.proto
index f3db71f..e827956 100644
--- a/proto/cache.proto
+++ b/proto/cache.proto
@@ -511,7 +511,7 @@
 }
 
 // Serialized form of com.google.gerrit.server.comment.CommentContextCacheImpl.Key
-// Next ID: 6
+// Next ID: 7
 message CommentContextKeyProto {
   string project = 1;
   string change_id = 2;
@@ -520,6 +520,8 @@
 
   // hashed with the murmur3_128 hash function
   string path_hash = 5;
+
+  int32 context_padding = 6;
 }
 
 // Serialized form of a list of com.google.gerrit.extensions.common.ContextLineInfo