chore: Add MFA example app

[tusharpandey13]

Add example app to demonstrate MFA APIs
Demonstrates

• Handling mfa_required errors.
• Listing available authenticators
• Making challenge request
• Getting access token after MFA Step Up
• Enrolling an authenticator

This is a stacked PR.
Current base: #2502

Usage

1. Populate .env.local file inside examples/mfa
2. Run pnpm run dev.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (feat/mfa-apis@a918539). Learn more about missing BASE report.

Additional details and impacted files

@@               Coverage Diff                @@
##             feat/mfa-apis    #2507   +/-   ##
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  Hits                     ?     4571           
  Misses                   ?     1383           
  Partials                 ?        7           

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