feature #9: Add routes for login

src/main/kotlin/fr/dcproject/repository/User.kt

@@ -0,0 +1,19 @@
+package fr.dcproject.repository
+
+import fr.postgresjson.connexion.Requester
+import fr.postgresjson.repository.RepositoryI
+import io.ktor.auth.UserPasswordCredential
+import fr.dcproject.entity.User as UserEntity
+
+class User(override var requester: Requester) : RepositoryI<UserEntity> {
+    override val entityName = UserEntity::class
+
+    fun findByCredentials(credentials: UserPasswordCredential): UserEntity? {
+        return requester
+            .getFunction("check_user")
+            .selectOne(
+                "username" to credentials.name,
+                "plain_password" to credentials.password
+            )
+    }
+}