Java: Add predicates to `InitializerMethod` for getting explicit initializer blocks

The CodeQL class `InitializerMethod` (and its subclasses `InstanceInitializer` and `StaticInitializer`) models the implicit initializer method initializing fields and containing the explicit initializer blocks.
However, it appears there is currently no easy way to get the explicit initializer blocks other than trying to inspect the `InitializerMethod` body which can be error-prone.

It might therefore be good to add the following two predicates for `InitializerMethod`:
- `getExplicitInitBlock(int index)`
- `getAnExplicitInitBlock()`