fix(gotrue)!: assert asyncStorage is provided for PKCE flow in the constructor

[gourabsingha1]

Root cause
When flowType is AuthFlowType.pkce (the default) and no asyncStorage is provided, calling getOAuthSignInUrl() or getLinkIdentityUrl() fails late at call time. To provide a better developer experience and fail fast, the misconfiguration should be validated up front in the constructor.

Fix
Added an assert in the GoTrueClient constructor to validate that asyncStorage != null if flowType == AuthFlowType.pkce.

Tests

• Added a unit test in gotrue/test/client_test.dart to verify that GoTrueClient throws an AssertionError if asyncStorage is missing when using PKCE flow.
• Updated all test suites in gotrue and supabase packages to pass a mock storage or specify implicit flow type where no storage was configured.

Fixes #1446

[Vinzent03]

As stated in your linked issue this is considered as a breaking change so won't plan this for main yet but later for v3 when we start actively working on it.