fix(auth, authenticator): prevent fetch session during sign up

[Jordan-Nelson]

Issue #, if available: #3122

Description of changes:

• prevent Auth.signIn() from fetching the session by using loadCredentials() insead of getUserPoolTokens() and by using an AnonymousCredentialsProvider on sign in calls.
• Update the Authenticator to use getCurrentUser to check the sign in state prior to calling sign in. This aligns with the updated behavior in the auth library and will prevent unnecessary network calls to fetch the session.

Background:

• The sign in flow can result in unnecessary network calls. At multiple points in the sign in flow there is an attempt to retrieve credentials. If there is no unauthenticated identity at this point, the auth lib attempts to create an unauthenticated identity. This results in 2 extra calls in the case where unuathenticated identities are allowed (one to get ID and one to get credentials) and results in several additional calls in the case that unuathenticated identities are not allowed (each attempt will call get ID which will fail).

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[dnys1]

Suggested change

	// initiateAuth does not require credentials
	// initiateAuth does not require credentials and using the default fetchAuthSession
	// credentials provider will trigger unnecessary calls to Cognito

[dnys1]

Suggested change

	(_) => throw StateError('.signIn() should not fetch auth session.'),
	(_) => fail('.signIn() should not fetch auth session.'),

[Jordan-Nelson]

Closing in favor of #3671