chore(webauthn): change user_id from uuid.UUID to string

[shentschel]

allows any string as user id

Related to: #5

[shentschel]

WIP: Need to rework how to get a list of credentials now

[FreddyDevelop]

Would be good if the change from uuid to string is backwards compatible.

[shentschel]

Hmm, is that should already be possible except for migrating down.

Do you have an example where this is not possible?

[FreddyDevelop]

Yes, when you created a credential with a uuid without the changes from this PR (e.g. v0.1.0) and then you want to login with the credential with the changes from this PR, then you get an error: failed to get user: failed to get webauthn user by user id: ERROR: invalid byte sequence for encoding \"UTF8\": 0xb8 (SQLSTATE 22021)

[shentschel]

Fixed the backward compatibility issue by trying to convert the userHandle into an uuid.
If it works: the uuid will then be converted into a string.
If not: the userHandle will directly be converted into a string

I also needed to overwrite the parsedRequest.Response.UserHandle because it is used to check if user ID and handle match in the webauthn lib