Populate proto_descriptors in GetDatabaseDdl response

[silas]

The proto_descriptors field isn't populated in the GetDatabaseDdl response, it'd be great to get that added.

cloud-spanner-emulator/frontend/handlers/databases.cc

Lines 280 to 294 in 7f22a69

	// Returns the schema of a database as a list of formatted DDL statements.
	absl::Status GetDatabaseDdl(RequestContext* ctx,
	const database_api::GetDatabaseDdlRequest* request,
	database_api::GetDatabaseDdlResponse* response) {
	ZETASQL_ASSIGN_OR_RETURN(std::shared_ptr<Database> database,
	GetDatabase(ctx, request->database()));
	absl::StatusOr<std::vector<std::string>> printed_statements =
	backend::PrintDDLStatements(database->backend()->GetLatestSchema());
	ZETASQL_RETURN_IF_ERROR(printed_statements.status());
	for (const auto& statement : *printed_statements) {
	response->add_statements(statement);
	}
	return absl::OkStatus();
	}