feat(auth0-auth-js): add multi-factor authentication (MFA) support

[amitsingh05667]

Summary

• New verify method - exchange MFA token + code for access tokens (otp, oob, recovery-code)
• Added clientSecret support to challengeAuthenticator
• Added getConfiguration option for openid-client integration
• OAuth2Error extended with mfa_token and mfa_requirements
• New isMfaRequiredError(error) type guard to detect mfa_required responses
• Exposes authClient.mfa
• Token errors now propagate mfa_token and mfa_requirements when Auth0 returns mfa_required

Design decisions

• MfaClient (src/mfa/): New verify method — exchange MFA token + code for access tokens (otp, oob, recovery-code); Added clientSecret support to challengeAuthenticator; Added getConfiguration option for openid-client integration
• errors.ts: OAuth2Error extended with mfa_token and mfa_requirements; New isMfaRequiredError(error) type guard to detect mfa_required responses
• AuthClient: Exposes authClient.mfa; Token errors now propagate mfa_token and mfa_requirements when Auth0 returns mfa_required

Test plan

• 33 unit tests covering all MFA operations (listAuthenticators, enrollAuthenticator, deleteAuthenticator, challengeAuthenticator, verify)
• verify method coverage flows
• npm run build passes

[nandan-bhat]

Approving this PR, as this the same PR (#164) isolating auth0-auth-js changes in a separate PR.