#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/resources/application.properties

@@ -28,7 +28,7 @@ spring.thymeleaf.encoding=UTF-8
 spring.web.resources.mime-mappings.webmanifest=application/manifest+json
 spring.mvc.async.request-timeout=${SYSTEM_CONNECTIONTIMEOUTMILLISECONDS:1200000}
 
-spring.datasource.url=jdbc:h2:file:./configs/stirling-pdf-DB-2.3.232;DB_CLOSE_DELAY=-1;DB_CLOSE_ON_EXIT=FALSE
+spring.datasource.url=jdbc:h2:file:./configs/stirling-pdf-DB-2.3.232;DB_CLOSE_DELAY=-1;DB_CLOSE_ON_EXIT=FALSE;MODE=PostgreSQL
 spring.datasource.driver-class-name=org.h2.Driver
 spring.datasource.username=sa
 spring.datasource.password=