feat(gotrue, supabase_flutter): Add signInWithSSO method

[dshukertjr]

What kind of change does this PR introduce?

Add signInWithSSO() method that can be called like the following:

await supabase.auth.signInWithSSO(
  domain: `company.com`
);

signInWithSSO() works in a similar way as signInWithOAuth() where gotrue-dart has a getSSOSignInUrl() that provides the sign in URL, and supabase_flutter handles the user redirect.

supabase-js implementation here: https://github.com/supabase/gotrue-js/blob/master/src/GoTrueClient.ts#L758

Closes #759