feature: Implement Serialized Entity Pattern (#2150)

* Add files:
- App.java
- Country.java
- CountryDao.java
- CountrySchemaSql.java
- CountryTest.java
- README.md
- serialize-entity.urm.puml
- pom.xml (inherit from parent pom.xml)
Update files:
- pom.xml (parent pom.xml, add 1 module called serialized-entity)

* Fix duplicated error in pom.xml

* Update:
- pom.xml (module servant, changed by error, now fixed)
- pom.xml (module serialized-entity)

* Update:
- pom.xml (changed, artifactId)

* Update:
- pom.xml

Add:
AppTest.java

* Resolved changes required by the reviewer

Co-authored-by: Ilkka Seppälä <iluwatar@users.noreply.github.com>