feat: abstract internal database access

[ymarcon]

Currently the internal database consists of a an embedded orientdb instance. The benefit of defining an abstraction layer would be:

• externalize the orientdb service so that it is not affected by opal's memory usage (vertical scalability) and it can be shared by several opal instances (horizontal scalability)
• use a different document-oriented database (like mongodb)
• make config flexible using a plugin discovered at runtime

[ymarcon]

Migrate to ArcadeDB https://blog.arcadedb.com/welcome-to-arcadedb https://github.com/ArcadeData/arcadedb https://docs.arcadedb.com/#OrientDB