feat: add email and sms integration identifier overrides

DVRL-68 mention push editor also supports system and payload variables

DVRL-81 email attachment should be either in buffer or base46 format

### What? User can send attachments file with email while triggering [https://docs.novu.co/channels-and-providers/email/overview#sending-email-attachments](https://docs.novu.co/channels-and-providers/email/overview#sending-email-attachments) ### Why? (Context) Novu allows only base64 or buffer format for attachments ### Definition of Done 1. mention this in above linked docs section with example

DVRL-80 using handlebar in email subject

DVRL-97 document how to use array length in email editor

DVRL-69 How to use headless for multiple users

We have headless.initializeSession method to generate token and initialize the session. If user log out and new user login, then calling again this method with second user subscriberId will generate new token and session. mention in docs this information in correct words

DVRL-77 how to override email integration in case of multi provider integrations

{ "to": "subscriberId", "payload" : {}, "overrides": { "email": { "integrationIdentifier": "sendgrid-DqJizp2Zt" } } }

DVRL-74 bell icon and notifcation center in novu web-app can not be used for testing

novuhq / docs

feat: add email and sms integration identifier overrides #377