add DenominationValidator to validate dosage fields in Prescription model

[sainak]

Proposed Changes

• Add DosageValidator to validate dosage char field

Associated Issue

• fixes The backend allows for any kind of string in the dosage field in the prescription. #1715

Merge Checklist

• Tests added/fixed
• Update docs in /docs
• Linting Complete
• Any other necessary step

Only PR's with test cases included and passing lint and test pipelines will be reviewed

@coronasafe/care-backend-maintainers @coronasafe/care-backend-admins

[codecov]

Codecov Report

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

Comparison is base (033d085) 60.75% compared to head (da914b9) 60.48%.

❗ Current head da914b9 differs from pull request most recent head b088601. Consider uploading reports for the commit b088601 to get more accurate results

Files	Patch %	Lines
care/facility/models/prescription.py	81.48%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1716      +/-   ##
==========================================
- Coverage   60.75%   60.48%   -0.28%     
==========================================
  Files         210      210              
  Lines       11533    11498      -35     
  Branches     1644     1636       -8     
==========================================
- Hits         7007     6954      -53     
- Misses       4285     4310      +25     
+ Partials      241      234       -7     

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[rithviknishad]

Maybe we could have made it more generic as DenominationValidator

and wherever dosage is there we could initialize the validator with:

dosage_validator = DenominationValidator(
  min=0.0001, 
  max=5000, 
  allowed_units={"mg", "g", "ml", "drop(s)", "ampule(s)", "tsp"},
)