[mlir][ods] Add support for custom directive in attr/type formats

This patch adds support for custom directives in attribute and type formats. Custom directives dispatch calls to user-defined parser and printer functions.

For example, the assembly format "custom<Foo>($foo, ref($bar))" expects a function with the signature

```
LogicalResult parseFoo(AsmParser &parser, FailureOr<FooT> &foo, BarT bar);
void printFoo(AsmPrinter &printer, FooT foo, BarT bar);
```

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D120944