fix: add explain remote query context (#120)

diff --git a/protos/engine/remote_engine.proto b/protos/engine/remote_engine.proto
index 1b97616..2270266 100644
--- a/protos/engine/remote_engine.proto
+++ b/protos/engine/remote_engine.proto
@@ -147,6 +147,8 @@
   Low = 1;
 }
 
+enum Explain { Analyze = 0; }
+
 message ExecContext {
   // This field is deprecated, use request_id_str instead.
   uint64 request_id = 1;
@@ -157,6 +159,7 @@
   QueryPriority priority = 5;
   string displayable_query = 6;
   string request_id_str = 7;
+  optional Explain explain = 8;
 }
 
 message ExecutePlanRequest {