#2270: External DB Support (#2457)

# Description

External DB support for Stirling PDF. You can now choose between the
default H2 or PostgreSQL by setting the new `enableCustomDatabase`
property to `true` or `false`.

To enable your own custom (PostgreSQL) database:
- Set `enableCustomDatabase` to `true`
- Add your database url to `customDatabaseUrl`
- Set your `username` and `password`

Closes #2270 

## Checklist

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have performed a self-review of my own code
- [x] I have attached images of the change if it is UI based
- [x] I have commented my code, particularly in hard-to-understand areas
- [ ] If my code has heavily changed functionality I have updated
relevant docs on [Stirling-PDFs doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
- [x] My changes generate no new warnings
- [x] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

src/test/java/stirling/software/SPDF/SPDFApplicationTest.java

@@ -0,0 +1,56 @@
+package stirling.software.SPDF;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.core.env.Environment;
+
+import stirling.software.SPDF.model.ApplicationProperties;
+import static java.nio.file.Files.createDirectories;
+import static java.nio.file.Files.createFile;
+import static java.nio.file.Files.delete;
+import static java.nio.file.Files.exists;
+
+@ExtendWith(MockitoExtension.class)
+public class SPDFApplicationTest {
+
+    @Mock
+    private Environment env;
+
+    @Mock
+    private ApplicationProperties applicationProperties;
+    
+    @InjectMocks
+    private SPDFApplication SPDFApplication;
+
+    @BeforeEach
+    public void setUp() {
+        SPDFApplication.setServerPortStatic("8080");
+    }
+
+    @Test
+    public void testSetServerPortStatic() {
+        SPDFApplication.setServerPortStatic("9090");
+        assertEquals("9090", SPDFApplication.getStaticPort());
+    }
+
+    @Test
+    public void testGetStaticPort() {
+        assertEquals("8080", SPDFApplication.getStaticPort());
+    }
+
+    @Test
+    public void testGetNonStaticPort() {
+        assertEquals("8080", SPDFApplication.getNonStaticPort());
+    }
+}