Add pad option to base64 encode/decode functions

[baranyildirim]

See #211

[divarvel]

Following discussions in #211, i think the best would be to always use URL_SAFE_NO_PAD and not add an option.

After testing with other implementations, changing the default behaviour will break other libs. So for now the best solution is to keep things as they are and manage base64 encoding manually if one wants to deviate from defaults.

[divarvel]

Given that other implementations depend on the padding, we should not modify default behaviour, and since the lib allows to manually manage base64 encoding, I will close this PR.

[baranyildirim]

accepting padded and non padded in deserialization, but only producing non padded base64 should be fine.

I don't understand how this is breaking compatibility, but I'll just maintain a separate fork. Thanks.

[divarvel]

since you can use Biscuit::from() with a byte slice directly, creating a helper function with the base64 settings you prefer would be way simpler than maintaining a fork imo