Allow define extra fields for sign up

[Plus17]

First of all, thank you very much for the work, I love this gem.

I would love to have the option to add a tos field to the sign up.

So this PR adds the extra_fields option in the sign_up config to be used by the method sign_up_params in the TokensController. I hope it is useful and can be included.

[melchua]

This would be super useful. Hope this can be reviewed/merged soon. I'm testing out the PR on my project.

[AzhirAhmadi]

Hi @nejdetkadir thanks for this great gem, Configuring the Devise for API was always a pain.

could you please update the RubyGems page's latest version too?
The latest version is 1.3 which does not contain this change, in the read me file you added eaxtra_fields to devise-api configs and it will raise an error since the gem from RubyGems (gem with latest version 1.3) does not have this change. I have to set the gem to the git repo main branch to fix this error.