feat: Implement add employee to project route (SCRUM-2) (!32)

Co-authored-by: Phan Huy Tran <p.tran@neusta.de>
Reviewed-on: #32
Reviewed-by: Jan Gleytenhoover <krisellp9@gmail.com>
Co-authored-by: Phan Huy Tran <ptran@noreply.localhost>
Co-committed-by: Phan Huy Tran <ptran@noreply.localhost>

requests/project/addEmployeeToProject.http

@@ -0,0 +1,2 @@
+POST http://localhost:8080/projects/1/employees/312
+Authorization: Bearer {{auth_token}}

requests/project/createProject.http

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+### GET request to example server
+POST http://localhost:8080/projects
+Authorization: Bearer {{auth_token}}
+Content-Type: application/json
+
+{
+  "name": "name",
+  "leading_employee": 1,
+  "employees": [2, 3],
+  "contractor": 4,
+  "contractor_name": "Peter File",
+  "comment": "goal of project",
+  "start_date": "01.01.2000",
+  "planned_end_date": "01.01.2001"
+}

requests/project/getAllProjects.http

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+### GET request to example server
+GET http://localhost:8080/projects
+Authorization: Bearer {{auth_token}}

requests/project/getProject.http

@@ -0,0 +1,3 @@
+### GET request to example server
+GET http://localhost:8080/projects/1
+Authorization: Bearer {{auth_token}}

requests/project/updateProject.http

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+### GET request to example server
+PUT http://localhost:8080/projects/1
+Authorization: Bearer {{auth_token}}
+Content-Type: application/json
+
+{
+  "name": "newName",
+  "leading_employee": 2,
+  "employees": [],
+  "contractor": 9,
+  "contractor_name": "New Contractor name",
+  "comment": "new goal of project",
+  "start_date": "01.01.2010",
+  "planned_end_date": "01.01.2021"
+}