added some service interfaces

SurveyLib.Core/Services/IAnswerService.cs

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+using SurveyLib.Core.Models;
+
+namespace SurveyLib.Core.Services;
+
+public interface IAnswerService
+{
+    Task<bool> AddAnswerAsync(Answer answer);
+    Task<bool> UpdateAnswerAsync(Answer answer);
+    Task<bool> DeleteAnswerAsync(int id);
+    Task<IEnumerable<Answer>> GetAnswersByCompletionIdAsync(int completionId);
+    Task<IEnumerable<Answer>> GetAnswersByQuestionIdAsync(int questionId);
+}

SurveyLib.Core/Services/ICompletionService.cs

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+using SurveyLib.Core.Models;
+
+namespace SurveyLib.Core.Services;
+
+public interface ICompletionService
+{
+    Task<bool> AddCompletionAsync(Completion completion);
+    Task<bool> UpdateCompletionAsync(Completion completion);
+    Task<bool> DeleteCompletionAsync(int id);
+    Task<IEnumerable<Completion>> GetCompletionsBySurveyIdAsync(int surveyId);
+}

SurveyLib.Core/Services/IQuestionService.cs

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+using SurveyLib.Core.Models;
+
+namespace SurveyLib.Core.Services;
+
+public interface IQuestionService
+{
+    Task<bool> AddQuestionAsync(QuestionBase question);
+    Task<bool> UpdateQuestionAsync(QuestionBase question);
+    Task<bool> DeleteQuestionAsync(int id);
+    Task<IEnumerable<QuestionBase>> GetQuestionsBySurveyIdAsync(int surveyId);
+}

SurveyLib.Core/Services/ISurveyService.cs

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+using SurveyLib.Core.Models;
+
+namespace SurveyLib.Core.Services;
+
+public interface ISurveyService
+{
+    Task<bool> AddSurveyAsync(Survey survey);
+    Task<bool> UpdateSurveyAsync(Survey survey);
+    Task<bool> DeleteSurveyAsync(int id);
+    Task<IEnumerable<Survey>> GetSurveysAsync();
+    Task<Survey> GetSurveyByIdAsync(int id);
+    Task<Survey> GetSurveyWithQuestionsByIdAsync(int id);
+    Task<Survey> GetSurveyWithAnswersByIdAsync(int id);
+}