Adopt to AOT changes in Commons

[onobc]

This code proposal adopts the newly added AOT changes (spring-data-commons/3282) to Cassandra repositories.

Note

While the AOT repositories support has not been added yet, I think it still makes sense to add these changes in as they will be required by AOT and this is a small enough change to focus on in a single PR.

• Introduces repository fragments contributor and updates the repository factories and factory beans w/ to use the default implementations.

• Also makes the CassandraRepositoryFactoryBean consistent with its reactive counterpart by only setting the mapping context from the CassandraOperations in afterPropertiesSet iff the mapping context was not specified by the user.

See: #1566

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[onobc]

Eventhough the fragment are empty by default for Cassandra, I chose to create a default impl that simple returned empty() to serve as a non-null fragmentsContributor default in the factory/factoryBeans as well as a placeholder in case we add default fragments (e.g. the MongoDB queryDsl fragments).