cache client instances in Auth0ClientFactory to prevent duplicate state

[subhankarmaiti]

Adds instance caching to Auth0ClientFactory (both native and web) so that multiple calls to createClient() with the same domain|clientId return the same IAuth0Client instance.

Problem

When Auth0Provider unmounts and remounts (e.g., navigation changes, React Suspense boundaries, or Fast Refresh), a brand-new client was created each time. This could lead to:

• Duplicate refresh-token exchanges racing against each other
• Inconsistent in-flight state between the Auth0 class singleton and the Auth0Provider hook
• Wasted native bridge allocations

Verification

• The Auth0 class and Auth0Provider now resolve to the same underlying client when configured with matching domain/clientId, ensuring credentials saved via one path are immediately visible to the other.