Count and Exists when using Constraints instead of method name queries

Nobody has claimed this yet.

#857 covered most of what is possible with method name queries to also be possible with constraint queries, but didn't include any alternative for countBy and existsBy.

One way to achieve this would be with `@Count` and `@Exists` annotations for repository methods where the former returns `long` or `int` and the latter returns `boolean`. Or, if a single annotation is preferred, then the return type could control whether it is a count vs. an exists operation. Aligning with the `@Find` annotation, we could even include a value to optionally indicate the entity type to disambiguate when using a multiple entity repository. Or, maybe someone will think of another way to design count/exists queries.

The general idea is that we could have something like,

```java
@Exists
boolean isAvailable(@By(_Product.ID) long productNumber,
@By(_Product.QUANTITY_IN_STOCK) @Is(AtLeast.class) int minAvailable);

@Count
long totalDiscounted(Department department, Restriction filter);
```

Start by reading issue #857 and the existing @Find annotation and repository method query handling. Compare the proposed @Count and @Exists designs, including return-type behavior and entity disambiguation for multiple-entity repositories. Done means an agreed API design with its behavior and required validation or tests specified.

Written by the indexing model from the issue text.