[FEAT] - Added model and form to add exhibitors

[Aaditya-Singh78]

Fixes #9111

Short description of what this resolves:

Added feature to manage exhibitors. It defines a model called Exhibitor with various fields - username, description, and website, allowing for the storage of exhibitor information, verifying exhibitors, updating their details.

Added test cases to validate the functionality of the exhibitor creation form, exhibitor modification form, and exhibitor instance creation.

Changes proposed in this pull request:

• Implementation of the Exhibitor model for managing exhibitor data
• Inclusion of methods within the Exhibitor model for verification and updating exhibitor details
• Creation of ExhibitorCreationForm and ExhibitorChangeForm forms for creating and modifying exhibitor instances
• Addition of test cases to validate the functionality of the exhibitor creation form, modification form, and exhibitor instance creation
• Verification of valid and invalid input scenarios to ensure the forms function correctly

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream development branch.
• The unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

• All the functions created/modified in this PR contain relevant docstrings.