Follow-up on HSEARCH-3222 In Progress .

The idea would be to provide a type-safe solution to building objects that encompass multiple projections, for example a "Color" object that requires a projection on the "red", "green" and "blue" fields.

In the future (not in this ticket), this will be particularly useful when combined with multi-valued nested object fields, allowing to get a List<Color> from a projection for example.

Draft: https://github.com/yrodiere/hibernate-search/tree/projections-syntax-attempt2 , https://github.com/yrodiere/hibernate-search/commit/5f9f56f587d3e44344b773bdefeb70c8fedf1be1

We decided to give up on the fluid API for composite projections, for two reasons:

1. We decided to remove the fluid APIs for predicates and sorts, so let's be consistent
2. Having one interface per number of parameters is complex to implement (or at least very redundant), and it also makes implementing extensions much more complex, because we'll have to implement each interface in each backend...