Allow to pass a custom Id

[josemrodrigu]

We are in the need of use our own id instead the one generated by the genId function.

I can see that useId accepts a default idprovided in props, but actually it's never used.

I can see that the only time this hook is executed is here, at the time that captionId is generated. Should it be possible to pass an id received in props, isn't it?
I think we can store this id (optional, of course) directly into DayPickerContext.

Let me know about your thoughts, I'll be glad to contribute.

Thanks!

[gpbl]

Implementing this in #1556