feat: send email notifications to DAU newly added to a proposal

[shivoomiess]

Description

This PR adds the feature to send email notifications to Data Access User (DAU) when they are newly added to a proposal.

Motivation and Context

This change is required to ensure that newly added DAUs are promptly notified about their addition to a proposal, thereby improving communication and efficiency.

Changes

1. An enum for 'DATA_ACCESS_USER_ADDED' has been added to the email template IDs.
2. A case for 'DATA_ACCESS_USERS_UPDATED' event has been added to the email handler. It sends emails to only the newly added users and skips if no new users are added.
3. An 'invitedUserIds' field has been added to the 'UserDataAccessUpdatedEvent' interface to keep track of newly invited users.
4. 'DataAccessUsersMutations' class has been updated to capture the current members before updating the data access users, to work out who is genuinely newly invited.

How Has This Been Tested

This has been tested by verifying that email notifications are sent out accurately to newly added DAUs, and no notifications are sent out in case of no new additions. Error handling has been implemented to log an error message if an email could not be sent.

The logs confirm this is the case:

Fixes Jira Issue

https://jira.ess.eu//browse/SWAP-5655

Depends On

Tests included/Docs Updated?

• I have added tests to cover my changes.
• All relevant doc has been updated

[jekabs-karklins]

LGTM - one minor change request.

Non-blocking thought: wiring this only into essEmailHandler means any other facility wanting the same notification has to reimplement a near-identical handler. Might be worth rethinking the email-handling structure so common cases live in one place while facilities can still add their own. Follow-up, not for this PR.