[JdbcIO] Allow fetchSize to be set for partitioned reads

[bvolpato]

The default for fetchSize is 50000 (https://github.com/apache/beam/blob/master/sdks/java/io/jdbc/src/main/java/org/apache/beam/sdk/io/jdbc/JdbcIO.java#L372), which is not even supported by some databases.

For example, DB2 limits to 32767 (source: https://knowledge.informatica.com/s/article/000205468?language=en_US).

This change will not only allow performance tuning when needed, but will enable usage of partitions on specific databases.

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[Abacn]

LGTM. thanks for the change. A followup could be expose this in xlang wrapper also