Provide connection validation/idle testing for Flink-Redis Connector

When idle connections are timed-out and closed by Redis server, the following exception will be thrown and the Flink job will eventually fail:

redis.clients.jedis.exceptions.JedisConnectionException: Unexpected end of stream. at redis.clients.util.RedisInputStream.ensureFill(RedisInputStream.java:199) at redis.clients.util.RedisInputStream.readByte(RedisInputStream.java:40) at redis.clients.jedis.Protocol.process(Protocol.java:151) at redis.clients.jedis.Protocol.read(Protocol.java:215)
......

It is clear that RedisCommandsContainerBuilder made use of GenericObjectPoolConfig without setting any parameters relating to idle testing. There are 2 possible solutions:

1. Replace GenericObjectPoolConfig directly with JedisPoolConfig, which enabled idle testing by default;
2. Make `testWhileIdle` configurable by user in FlinkJedisPoolConfig.

 Also, `testOnBorrow` and `testOnReturn` can be provided for those who want to validate connections.