optional last_sign_in_at in UserIdentity type

[lmoj]

What kind of change does this PR introduce?

Bug fix

What is the current behavior?

supabase.auth.get_user raises

gotrue.errors.AuthRetryableError: 1 validation error for UserResponse
user -> identities -> 0 -> last_sign_in_at
  field required (type=value_error.missing)

when the user hasn't signed in before

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (736f334) 37.59% compared to head (6fef36a) 37.59%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #265   +/-   ##
=======================================
  Coverage   37.59%   37.59%           
=======================================
  Files          25       25           
  Lines        2285     2285           
=======================================
  Hits          859      859           
  Misses       1426     1426           

Impacted Files	Coverage Δ
gotrue/types.py	100.00% <100.00%> (ø)

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