feat: unsubscribe to the newsletter by email

[Heba-WebDev]

Description

Closes #404

Changes proposed

• Added PATCH /api/v1/news-letter/unsubscribe Endpoint:
• Implemented a new endpoint to unsubscribe a newsletter subscriber by their email address.
• This endpoint uses a PATCH request and marks the subscriber as deleted by setting IsDeleted
• Created Commands and Handlers:
• Added DeleteSubscriberByEmailCommand and DeleteSubscriberByEmailCommandHandler for the PATCH endpoint.
• Wrote unit tests for DeleteSubscriberByEmailCommandHandler in the Hng.Application.Test project.

Check List (Check all the applicable boxes)

• My code follows the code style of this project.
• This PR does not contain plagiarized content.
• The title and description of the PR is clear and explains the approach.
• I am making a pull request against the dev branch (left side).
• My commit messages styles matches our requested structure.
• My code additions will fail neither code linting checks nor unit test.
• I am only making changes to files I was requested to.

Screenshots/Videos

• Subscriber successfully deleted
  
• Subscriber already deleted
  
• Subscriber not found
  
• Invalid email format