Added GetDrugExposureEndDate function

[Jay-sanjay]

This PR is related to #69

PR CheckList

• [x] Added GetDrugExposureEndDate function
• [x] Added doc-strings and test-suite for it
• [x] Added a multiple function dispatch

closes #69

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (ac5bcf0) 74.51% compared to head (7edede7) 74.37%.

Files	Patch %	Lines
src/getters.jl	70.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #71 +/- ## ========================================== - Coverage 74.51% 74.37% -0.15% ========================================== Files 6 6 Lines 310 320 +10 ========================================== + Hits 231 238 +7 - Misses 79 82 +3 ```

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[Jay-sanjay]

@TheCedarPrince yes that might be a good idea,I can file an issue for it. Because if in case I won't able to add that for all function, then also the issue would be present for the future work.

That said thanks for the review, it means a lot to me