Fix the auth helpers to only require fields they need from `AuthIdentity`

[infomiho]

The auth helpers like getEmail and getUsername didn't work unless you provided the full user object with full AuthIdentity objects in the identities array.

Given that we suggest to users to include only the providerName and providerUserId when manually fetching the user + auth + identities, we should make sure our auth helpers work in that scenario.

This PR:

• makes it so that they only need to pass in the providerName and providerUserId when using the helpers.
• includes a headless test to ensure this helper works in the future.

[infomiho]

Good point! The tricky part with the UserEntityWithAuth name is that it is correct in a sense that it represents a tree of full Prisma entities. The second type is derived from it by making it as minimal as possible to make the function as flexible as possible.

The problem is that the user will see this wacky (albeit correct) name, and we want to do better.

Some alternatives:

• internal: CompleteUserEntityWithAuth, public: UserEnittyWithAuth
• internal AuthEntityTree, public: UserEnittyWithAuth
• internal: UserEntityWithAuthFields, public: UserWithRequiredAuthFields

I maybe like the first option the best since it's just a Complete vs nothing which gives us the relationship you suggested.