Add support for external/custom marshalling/unmarshaling implementations

Problems with the current state:

Currently, Spring Data Redis allows marshalling (serializing) POJOs using Redis Hashes with @RedisHash annotation. This approach doesn’t allow efficiently store and querying nested objects like discussed here. Also, the selection of the proper serializer for POJOs is confusing for the developers.

Proposed solution:

Introduce a new API that allows extending Spring Data Redis with external/custom marshalling/unmarshaling implementations.

For example, it will allow us to build JSON-based serialization to store POJOs as JSON documents in Redis. JSON.GET allows to retrieve and update embedded data efficiently with JSONPath, which can be used to significantly improve the performance of retrieving nested objects.

An example of a JSON-based serialization can be found here https://github.com/redis/redis-om-spring/blob/main/demos/roms-documents/src/main/java/com/redis/om/documents/domain/Company.java#L27

This can be combined with https://github.com/spring-projects/spring-data-redis/issues/2546 to allow indexing JSON documents in addition to Hashes.

Start by reviewing the proposed JSON-based serialization example in Company.java and the related spring-data-redis issue 2546. Define the external marshalling and unmarshalling API boundaries and how they interact with Redis JSON documents and existing Hash-based storage; done means the design supports the stated nested-object use case and its indexing integration.