Add "queryForFirst" methods to JdbcTemplate for querying first row of ResultSet

[quaff]

It's very common to get first row of ResultSet or null for empty ResultSet, for example JdbcStepExecutionDao::getLastStepExecution from Spring Batch, see https://github.com/spring-projects/spring-batch/pull/4798 for background.

We should call Statement::setMaxRows as hints (some legacy driver may not honer it) before executing, and only consume first row of the ResultSet. Not sure about the return type, @Nullable T or Optional<T> is better?

If this proposal is accepted, then Optional<T> first() should be added to JdbcClient.

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1. Why not queryForList ? It will load all rows into memory.

2. Why not queryForObject ? It requires exactly one row, but the ResultSet may be empty, and there is no standard syntax to limit one row across all databases.

3. Why not queryForStream().findFirst() ? The result stream need be closed explicitly, and maxRows is set for whole JdbcTemplate, we need specific 1 only for this query.