[mlir] Add isa/dyn_cast support for dialect interfaces

This matches the same API usage as attributes/ops/types. For example:

```c++
Dialect *dialect = ...;

// Instead of this:
if (auto *interface = dialect->getRegisteredInterface<DialectInlinerInterface>())

// You can do this:
if (auto *interface = dyn_cast<DialectInlinerInterface>(dialect))
```

Differential Revision: https://reviews.llvm.org/D117859