Collection of improvements to the fluent API

[schauder]

This issue is a portfolio wide collection of issues related to fluent API enhancements.
The idea is to collect them here in order to check for synergies and common patterns.

JPA

• Allow fluent API to reuse existing named entity graphs spring-data-jpa#4083

Mongo

• Add projection support to SpringDataMongodbQuery [DATAMONGO-2563] spring-data-mongodb#3417
• PR on ReactiveGridFsOperations for default methods accepting CriteriaDefinition [DATAMONGO-2519] spring-data-mongodb#3374

Relational

• Add projection support (and fluent API) to JdbcAggregateTemplate spring-data-relational#2109
• Explore infrastructure for Dialect-specific Criteria spring-data-relational#2076