[DMN Designer] Search feature - Define search architectural approach

This JIRAs refers to the discussions for different implementation approaches that can be adopted for supporting GDT editor and DMN editor.

With this JIRA done, two initial implementations will be triggered:

• https://issues.jboss.org/browse/DROOLS-4212 (DMN graph PoC)
• https://issues.jboss.org/browse/DROOLS-4211 (GDT PoC)

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Definitions:

https://gist.github.com/karreiro/a43c1a3e5cd0d2811ac44f5cc7ccc285