Add `jdbcAggregateOperationsRef` to `@EnableJdbcRepositories`

[kota65535]

fix: #544, #687, #994

This PR adds jdbcAggregateOperationsRef to @EnableJdbcRepositories that enables to specify JdbcAggregateOperations bean name to be used for creating repositories, which significantly simplifies the configuration for multiple repositories. I've also created a sample project for testing this feature. Multiple repositories with different dialects can be configured using two configurations (Db1Config.java, Db2Config.java). As you can see, creating the JdbcAggregateOperations bean is still a bit messy. It may be better to have some factory class for creating them.

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[kota65535]

Hi @mp911de @schauder, could you please take a look?

[kota65535]

@Mikhail2048 Thanks for your review. I've fixed the points you mentined, please check it.

[kota65535]

@mp911de @schauder Could this PR be included to 3.3 M2? Thank you!

[kota65535]

@mp911de @schauder @Mikhail2048 Could you please review when you have time?