feat: vector index delete item rpc

This PR adds an RPC to delete items from the vector index by id. The
semantics are to delete any items with an id in the provided list.

proto/vectorindex.proto

@@ -4,6 +4,7 @@ package vectorindex;
 
 service VectorIndex {
     rpc AddItemBatch(_AddItemBatchRequest) returns (_AddItemBatchResponse) {}
+    rpc DeleteItemBatch(_DeleteItemBatchRequest) returns (_DeleteItemBatchResponse) {}
     rpc Search(_SearchRequest) returns (_SearchResponse) {}
 }
 
@@ -22,6 +23,14 @@ message _AddItemBatchResponse {
     repeated uint32 error_indices = 1;
 }
 
+message _DeleteItemBatchRequest {
+    string index_name = 1;
+    repeated string ids = 2;
+}
+
+message _DeleteItemBatchResponse {
+}
+
 message _Vector {
     repeated float elements = 1;
 }