feat: implement core use cases and DTOs (OAuth2, tests, and docs pending)

domain/src/main/java/com/pablotj/restemailbridge/domain/model/Email.java

@@ -0,0 +1,15 @@
+package com.pablotj.restemailbridge.domain.model;
+
+import lombok.Builder;
+import lombok.Getter;
+
+@Getter
+@Builder
+public class Email {
+
+    private String from;
+    private String to;
+    private String subject;
+    private String body;
+
+}

domain/src/main/java/com/pablotj/restemailbridge/domain/repository/EmailRepository.java

@@ -0,0 +1,8 @@
+package com.pablotj.restemailbridge.domain.repository;
+
+import com.pablotj.restemailbridge.domain.model.Email;
+
+public interface EmailRepository {
+
+    Email save(Email email);
+}

domain/src/main/java/com/pablotj/restemailbridge/domain/service/EmailService.java

@@ -0,0 +1,8 @@
+package com.pablotj.restemailbridge.domain.service;
+
+import com.pablotj.restemailbridge.domain.model.Email;
+
+public interface EmailService {
+
+    Email sendEmail(Email email);
+}