Add stdin support?

[imathews]

It would be quite helpful if the execute_query binary supported reading queries from stdin – especially for very long queries where we might hit system argv limits. Something like echo "SELECT 1 + 1" | execute_query -.

This would only require a minor change to execute_query.cc to read from stdin. Would you accept a PR? The relevant code changes are also below, which I've tested and validated on my own fork:

// Added to includes

#include <sstream>

// Updated RunTool fn:

absl::Status RunTool(const std::vector<std::string>& args) {
  ExecuteQueryConfig config;

  GOOGLESQL_RETURN_IF_ERROR(InitializeExecuteQueryConfig(config));

  GOOGLESQL_ASSIGN_OR_RETURN(std::unique_ptr<ExecuteQueryWriter> writer,
                   MakeWriterFromFlags(config, std::cout));

  if (absl::GetFlag(FLAGS_web)) {
    return RunExecuteQueryWebServer(absl::GetFlag(FLAGS_port));
  }

  std::string sql;
  if (args.size() == 1 && args[0] == "-") {
    // Read SQL from stdin
    std::ostringstream ss;
    ss << std::cin.rdbuf();
    sql = ss.str();
  } else {
    sql = absl::StrJoin(args, " ");
  }
  return ExecuteQuery(sql, config, *writer);
}

// Updated usage string:

const char kUsage[] =
    "Usage: execute_query "
    "{ \"<sql>\" | - | {--web [--port=<port>] } }\n";