feat: add ddl task service (#53)

* feat: add ddl task service

* chore: apply suggestions from CR

* chore: add path to build.rs

Author: WenyXu

build.rs

@@ -29,6 +29,7 @@ fn main() {
                 "proto/greptime/v1/meta/common.proto",
                 "proto/greptime/v1/meta/heartbeat.proto",
                 "proto/greptime/v1/meta/route.proto",
+                "proto/greptime/v1/meta/ddl.proto",
                 "proto/greptime/v1/meta/store.proto",
                 "proto/greptime/v1/meta/lock.proto",
                 "proto/greptime/v1/meta/cluster.proto",