Add helper method for getting proposition condition

### Feature description

In propositions that have conditions, we should add a helper method to get the condition

### Use case

* `objectCondition`, `objectTumorType`, and `conditionQualifier` are all condition field names on Propositions. I want to be able to call `Proposition.get_condition()` without having to know the name of the field

### Proposed solution

_No response_

### Alternatives considered

_No response_

### Implementation details

_No response_

### Potential Impact

_No response_

### Additional context

_No response_

### Contribution

Yes, I can create a PR for this feature.