Add tokenForConnection method to OAuth

Description

This PR adds a new tokenForConnection method to the OAuth class that allows exchanging a refresh token for a federated connection access token. This is particularly useful when you need to access a social provider's API using the user's refresh token.

This PR also removes enforcing namespace ownership requirements for token types in custom token exchange (this is not required to be done on SDK side)

References

• RFC 8693: OAuth 2.0 Token Exchange

Changes

• Added TokenForConnectionOptions interface with required refreshToken and connection parameters, and optional loginHint
• Added constants for grant type, token types, and endpoint URL
• Implemented tokenForConnection method with proper parameter validation and error handling
• Added comprehensive test suite covering success and error scenarios
• Removed logic to enforce namespace ownership requirements for token types from customTokenExchange

Manual Testing Steps

1. Initialize the OAuth client with your Auth0 credentials:

const oauth = new OAuth({
  domain: 'your-domain.auth0.com',
  clientId: 'your-client-id',
  clientSecret: 'your-client-secret'
});

2. Call tokenForConnection method:

const response = await oauth.tokenForConnection({
  refreshToken: 'valid-refresh-token',
  connection: 'google-oauth2'
});
console.log(response.data.access_token);

Test Results

PASSING

Test Suites: 48 passed, 48 total
Tests:       1569 passed, 1569 total
Snapshots:   0 total
Time:        6.928 s, estimated 7 s
Ran all test suites.