Replace Gson by Jackson on legal tag creation in LegalService

Resolves the issue: [Bug] 'expirationDate' field is serialized incorrectly due to the use of Gson

1. Add ObjectMapper to LegalService.
2. Change HttpRequest.post(<Object>) by HttpRequest.post().body(<String>) in LegalService using ObjectMapper.
3. Add LegalServiceTest to check the field expirationDate serializes correctly.

Changes include

• Refactoring
• Bugfix (non-breaking change that solves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• Added Unit Tests, wherever applicable.
• Updated the Readme
• Existing Tests passed
• Verified functionality locally
• Self Reviewed my code for formatting and complex business logic.