Adding a validation for the required email field in p12 credentials form

[aneveux]

Hello 👋

While creating a credential using a p12 key, there is no validation of the email field while it should actually be required.

This PR adds a simple form validation adding an error message when no value is specified for the email address.

Here is how the validation looks:

Validation Failure

Validation Success

The feature was tested manually using mvn hpi:run.

Testing done

Submitter checklist

Preview Give feedback

1. Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
2. Ensure that the pull request title represents the desired changelog entry
3. Please describe what you did
4. Link to relevant issues in GitHub or Jira
5. Link to relevant pull requests, esp. upstream and downstream changes
6. Ensure you have provided tests - that demonstrates feature works or fixes the issue

[Pldi23]

Overkill IMO, but ok

[amuniz]

LGTM