Introduce the JdbcCriteriaExecutor interface for Criteria queries

[mipo256]

In Spring Data JPA we have the JpaSpecificationExecutor, which allows to enhance the functionality of concrete repositories by adding findAll() with Specification methods and its siblings.

Currently, if the user wants to utilize the CriteriaDefintion API, he/she needs to use the JdbcAggregateTemplate. It is great, but I think, it might be a good idea to introduce the JdbcCriteriaExecutor or something like that in order to allow the users to benefit from CriteriaDefinition and its API by working solely on the repository level.

The CriteriaDefinition, although, not being as powerful as Specification API in the Spring Data JPA, is nevertheless the approach to build the queries with the dynamic WHERE conditions. So I think by adding the JdbcCriteriaExecutor we will align the approach in Spring Data JDBC with Spring Data JPA and make the usage of CriteriaDefinition more intuitive for end users.

WDYT @mp911de @schauder ? I can assist here with an implementation.

[mp911de]

Introducing the Specification executor for JPA was a neat idea from a DDD perspective but it broke our design principle to not require store-specific API on the caller side. Specifications use the JPA API and therefore they render a repository non-portable to other technologies.

Querydsl and Query-by-Example are Spring Data-specific and supported by a number of repositories.

Introducing an executor for Criteria would introduce again something that we want to avoid and therefore, we don't want to include such a feature. In any case, thanks for the suggestion.