[mlir][ODS] Add support for passing properties to `custom`

Printing and parsing properties of ops is currently only possible through the `prop-dict` attribute. This forces a specific place that the property is printed at and is generally not very flexible.

This patch adds support for passing properties to the `custom` directive, making it possible to incorporate them with more complex syntax. This makes it possible to parse and print them without generic syntax and without the use of `prop-dict`.

Differential Revision: https://reviews.llvm.org/D155072