feat: add option to wait on session pool creation

[thiagotnunes]

Adds option to wait for min sessions to be populated in the session pool before returning the database client back to the user. This only done during the database client creation and it is useful for benchmarking.

Example usage:

SpannerOptions options = SpannerOptions
    .newBuilder()
    .setSessionPoolOption(SessionPoolOptions
        .newBuilder()
        .setMinSessions(100)
        .setMaxSessions(400)
        .setWaitForMinSessions(Duration.ofMinutes(5))
        .build())
    .build();
try (Spanner spanner = options.getService()) {
  // This call will wait (at most) 5 minutes for the session pool creation to have 100 sessions. A SpannerException
  // will be thrown if the session pool does not have the min sessions within the wait duration.
  DatabaseClient client = spanner.getDatabaseClient(DatabaseId.of("my-project", "my-instance", "my-database"));

  try (ResultSet resultSet = client.singleUse().executeQuery(Statement.of("SELECT 1"))) {
    if (resultSet.next()) {
      System.out.println(resultSet.getLong(0));
    }
  }
}

[thiagotnunes]

0 minSessions always succeed, so we increment it by one