Add return_components support to R-learner

[aman-coder03]

Proposed changes

this PR adds return_components support to the R-Learner, bringing its API in line with the existing T- and X-Learner implementations

specific changes..

• adds a return_components argument to predict() and fit_predict() for both BaseRLearner and BaseRClassifier
• returns the nuisance components used by the R-Learner
  • yhat: outcome model predictions (E[Y|X])
  • p: propensity score estimates (E[W|X])
• adds the same mutual exclusion guard as other meta-learners, preventing return_ci and return_components from being used together
• fits the nuisance outcome model after cross-validation so that it can be used for inference-time component retrieval.
• adds tests covering the new return_components functionality for both predict() and fit_predict() along with the mutual exclusion behavior

fixes #304

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