Added GetDrugExposureStartDate Function

[nitverse]

Added the function getDrugExposureStartDate as mentioned in the issue #68

[codecov[bot]]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (ac5bcf0) 74.51% compared to head (54c4504) 73.33%.

Files	Patch %	Lines
src/getters.jl	0.00%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #70 +/- ## ========================================== - Coverage 74.51% 73.33% -1.19% ========================================== Files 6 6 Lines 310 315 +5 ========================================== Hits 231 231 - Misses 79 84 +5 ```

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[TheCedarPrince]

Hi @nitverse,

Thanks for giving this a try! There's still a fair amount of work to do on this as of now, but happy to work with you through it! Here's what else needs to be done:

• [ ] Add an execution dispatch for this function which executes the code against the database
• [ ] Add a dispatch which accepts a dataframe object
• [ ] Add a reference to the function within the documentation
• [ ] Add tests for all the dispatches

Thankfully, we've enumerated this procedure in a pretty straightforward manner and would suggest looking at the code example here: https://github.com/JuliaHealth/OMOPCDMCohortCreator.jl/blob/ac5bcf02a67ff0afe7868c6d77b61674daa2795e/src/getters.jl#L1485 and referring to the PR I mentioned in the original issue.

Let me know if you have any questions!

[TheCedarPrince]

Hey @nitverse -- were you going to work more on this PR?

[Jay-sanjay]

This is fixed by #72