Add FNet Encoder Layer

[abheesht17]

@mattdangerw , @chenmoneygithub and the rest of the keras-nlp team:

The FNet paper, which came out last year, uses FFTs instead of attention and achieves 95% of BERT's performance on various downstream tasks. It would be great if we could add this to the library. Simultaneously, a text classification example can be added (using a few FNet Encoder layers). Would love to take this up, if the team is interested :).

[chenmoneygithub]

Yes, your contribution is more than welcome! Thanks!