feat: Add Custom Token Exchange support

[subhankarmaiti]

Implements Custom Token Exchange (RFC 8693) for auth0-server-python, enabling token exchange from external identity providers and legacy systems without browser redirects.

Changes

New Methods

• custom_token_exchange() - Exchanges custom tokens for Auth0 tokens without session management (utility method)
• login_with_custom_token_exchange() - Exchanges custom tokens AND establishes user session (high-level login method)

New Types

• CustomTokenExchangeOptions - Configuration for token exchange
• LoginWithCustomTokenExchangeOptions - Configuration for login with exchange
• TokenExchangeResponse - Token exchange response with access/ID/refresh tokens
• LoginWithCustomTokenExchangeResult - Login result with session state

Error Handling

• CustomTokenExchangeError - Specific exception for token exchange failures
• CustomTokenExchangeErrorCode - Error codes (INVALID_TOKEN_FORMAT, MISSING_ACTOR_TOKEN_TYPE, TOKEN_EXCHANGE_FAILED, INVALID_RESPONSE)

Use Cases

1. Backend Token Exchange (No Session)

response = await client.custom_token_exchange(
    CustomTokenExchangeOptions(
        subject_token="external-token",
        subject_token_type="urn:acme:mcp-token",
        audience="https://api.example.com"
    )
)

2. User Login via Token Exchange (With Session)

result = await client.login_with_custom_token_exchange(
    LoginWithCustomTokenExchangeOptions(
        subject_token="legacy-system-token",
        subject_token_type="urn:acme:legacy-token",
        audience="https://api.example.com"
    ),
    store_options={"request": request, "response": response}
)