Fallback implementation for shortest path between two nodes when GDS is not available

[CatoLeanTruetschel]

Fallback implementation for shortest path between two nodes when GDS is not available. This can be a slow in-memory implementation as this is only a fallback. The detection whether GDS is available shall be done dynamically via requesting the database, without any configuration needed. The choice can be overriden via configuration.

Therefore possible config settings are:

• Auto
• In-memory
• Database-native

An exception will be thrown if the configuration sepcified database-native and the GDS plugin is not available.