#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/main/java/stirling/software/SPDF/config/interfaces/DatabaseInterface.java

@@ -0,0 +1,17 @@
+package stirling.software.SPDF.config.interfaces;
+
+import java.sql.SQLException;
+import java.util.List;
+
+import stirling.software.SPDF.model.provider.UnsupportedProviderException;
+import stirling.software.SPDF.utils.FileInfo;
+
+public interface DatabaseInterface {
+    void exportDatabase() throws SQLException, UnsupportedProviderException;
+
+    void importDatabase();
+
+    boolean hasBackup();
+
+    List<FileInfo> getBackupList();
+}