[mlir][OpFormatGen] Add support for specifiy "custom" directives.

This revision adds support for custom directives to the declarative assembly format. This allows for users to use C++ for printing and parsing subsections of an otherwise declaratively specified format. The custom directive is structured as follows:

```
custom-directive ::= `custom` `<` UserDirective `>` `(` Params `)`
```

`user-directive` is used as a suffix when this directive is used during printing and parsing. When parsing, `parseUserDirective` will be invoked. When printing, `printUserDirective` will be invoked. The first parameter to these methods must be a reference to either the OpAsmParser, or OpAsmPrinter. The type of rest of the parameters is dependent on the `Params` specified in the assembly format.

Differential Revision: https://reviews.llvm.org/D84719