Add item reader for Redis

[fmbenhassine]

This issue is to add an item reader for Redis based on Spring Data Redis. The tricky part is how to define the set of keys that the reader should iterate over:

• KEYS/SCAN are the most obvious choices (as Spring Data Redis provides iterable cursor-based APIs for these commands)
• Provide a key for which the value is an iterable data structure (list, set, stream, etc) and make the reader return items from that value

This is key to correctly design and implement the item reader (or readers?) for Redis.

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Reference: https://stackoverflow.com/questions/24703753/does-spring-batch-have-a-redis-itemreader-in-the-roadmap

[fmbenhassine]

RedisTemplate#scan is the API that fits well with an item reader as it returns a cursor over a set of keys specified by a ScanOption. Therefore, the new RedisItemReader should be based on those APIs.

As there is no iterable common base type for ListOperations, SetOperations, etc provided by Spring Data Redis, there is no plan for now to create an item reader per data structure (RedisListItemReader, RedisSetItemReader, etc). This is left to the user to implement in a custom reader.