Adding EEG example: SSVEPFormer for BCI data classification

[okbalefthanded]

Adding a New Keras 3 Timeseries example:

I prepared a tutorial for the classification of EEG data generated by Brain-Computer Interface experiment, the model i reproduced is considered the first Transformer based model for this type of data. available on this preprint Paper.

A PR is on the way right after opening this issue.