Add strategy to manage different FHIR condition models

As mentioned in FM-64, OpenMRS may store conditions as OpenMRS obs. However, someone else may have written their own table to store them seperately, so we need to be able to support folks who want to use either of these to generate their FHIR resource.

To do this, we need to develop a FHIR strategy pattern.
Look at the strategy pattern implementation in FHIR allergy intolerance resource, and develop similar classes for conditions.

Please note that while allergy intolerance has three strategies:

ObsAllergyStrategy.java
OpenMRS20AllergyStrategy.java
LegacyAllergyStrategy.java

We curently need to createonly one strategy -
ObsConditionStrategy.java

Anyone who wants additional strategies will be able to write their own strategy classes later.

Note: This task does not include writing any business logic for modeling FHIR condition resources