Add storage provider interface for parallel instance support

[Copilot]

Multiple OIDCClient instances cannot run in parallel as they overwrite each other's tokens in shared localStorage/sessionStorage.

Changes

Storage Provider Interface

• StorageProvider interface with getItem, setItem, removeItem methods supporting sync/async operations
• BrowserStorageProvider: sessionStorage for access tokens, localStorage for refresh tokens, configurable token names
• InMemoryStorageProvider: Map-based storage, no persistence

OIDCClient Updates

• Accepts storageProvider and tokenNames options
• All storage operations abstracted through provider interface
• verifyTokenValidity now async to support async providers
• Defaults to BrowserStorageProvider with standard token names (backward compatible)

Usage

Different storage providers:

const client1 = new OIDCClient(); // Browser storage (default)
const client2 = new OIDCClient({ 
  storageProvider: new InMemoryStorageProvider() 
});

Different token names:

const client1 = new OIDCClient({ 
  tokenNames: { token: "token1", refreshToken: "refreshToken1" }
});
const client2 = new OIDCClient({ 
  tokenNames: { token: "token2", refreshToken: "refreshToken2" }
});

Custom provider:

class MongoDBStorageProvider implements StorageProvider {
  async getItem(key: string) { /* ... */ }
  async setItem(key: string, value: string) { /* ... */ }
  async removeItem(key: string) { /* ... */ }
}

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