[YSQL] Introduce the PGConn::FetchAll helper method

[d-uspenskiy]

Jira Link: DB-7889

Description

There are lots of unit tests which explicitly checks number of rows/columns in fetch result.

  PGResultPtr res = ASSERT_RESULT(conn_->Fetch(query));
  ASSERT_EQ(PQntuples(res.get()), 3);
  ASSERT_EQ(PQnfields(res.get()), 3);
  std::array<int, 3> values = {
    ASSERT_RESULT(GetInt32(res.get(), 0, 1)),
    ASSERT_RESULT(GetInt32(res.get(), 1, 1)),
    ASSERT_RESULT(GetInt32(res.get(), 2, 1)),
  };
  ASSERT_EQ(values[0], 0);
  ASSERT_EQ(values[1], 100);
  ASSERT_EQ(values[2], -5);

The PGConn class has the FetchMatrix method which can reduce some boiler plate code.

  PGResultPtr res = ASSERT_RESULT(conn_->FetchMatrix(query, 3, 3));
  std::array<int, 3> values = {
    ASSERT_RESULT(GetInt32(res.get(), 0, 1)),
    ASSERT_RESULT(GetInt32(res.get(), 1, 1)),
    ASSERT_RESULT(GetInt32(res.get(), 2, 1)),
  };
  ASSERT_EQ(values[0], 0);
  ASSERT_EQ(values[1], 100);
  ASSERT_EQ(values[2], -5);

But it will be good to have additional helper method which reads all the rows as a vector of tuples. This helper method will help to simplify code even more

const auto values = ASSERT_RESULT((conn_->FetchAll<char, int>(query)));
decltype(values) expected_values = {{'a', 0}, {'b', 100}, {'y', -5}};
ASSERT_EQ(values, expected_values);

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