feat: MFA (#12290)

## Linear:
- https://github.com/chatwoot/chatwoot/issues/486

## Description
This PR implements Multi-Factor Authentication (MFA) support for user
accounts, enhancing security by requiring a second form of verification
during login. The feature adds TOTP (Time-based One-Time Password)
authentication with QR code generation and backup codes for account
recovery.

## Type of change

- [ ] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

- Added comprehensive RSpec tests for MFA controller functionality
- Tested MFA setup flow with QR code generation
- Verified OTP validation and backup code generation
- Tested login flow with MFA enabled/disabled

## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

---------

Co-authored-by: Pranav <pranav@chatwoot.com>
Co-authored-by: Sojan Jose <sojan@pepalo.com>
Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/services/base_token_service.rb

@@ -0,0 +1,27 @@
+class BaseTokenService
+  pattr_initialize [:payload, :token]
+
+  def generate_token
+    JWT.encode(token_payload, secret_key, algorithm)
+  end
+
+  def decode_token
+    JWT.decode(token, secret_key, true, algorithm: algorithm).first.symbolize_keys
+  rescue JWT::ExpiredSignature, JWT::DecodeError
+    {}
+  end
+
+  private
+
+  def token_payload
+    payload || {}
+  end
+
+  def secret_key
+    Rails.application.secret_key_base
+  end
+
+  def algorithm
+    'HS256'
+  end
+end