Added custom auth code options for google

[gregadams4]

Might just be missing something, but couldn't find a way currently to set access_type=offline to get a refresh token for this provider

[bentranter]

@gregadams4 Thanks for bringing this up! I we should instead set access_type=offline by default. As stated in Google's documentation,

Set the value to offline if your application needs to refresh access tokens when the user is not present at the browser.

I feel like that covers the authentication use case for Goth. We also used to set it by default on the old gplus provider, so I think it'd be better to have that as the default here for consistency.

[jmburges]

🤞 this gets merged soon!

[uded]

Can someone check, fix and merge this? This is a crucial update from the perspective of using Google auth for something more than just login...
This or #396 -- whatever seems to be better for goth.

[uded]

After using a newly merged #396 I have to say this would be a bit better. With a default for access_type=offline. But sometimes I do need to add refresh_token or something like that...