Refactor project architecture based on tech meeting

[MostafaTech]

• Use Application Services instead of mediatR
• Add dtos to application layer
• Add dtos to api layer
• Add fluent validation

Closes #112

[MostafaTech]

Adding FluentValidation to dtos and asp.net pipeline.
It uses ApiResult object like other api responses and will create results as below when validation fails:

{
  "data": null,
  "errorType": "validation",
  "errors": {
    "Name": [
      "'Name' must not be empty."
    ]
  },
  "isSuccess": false
}