feat: add string support for FreeParameterExpressions

[math411]

Issue #, if available:

Description of changes:
feat: add string support for FreeParameterExpressions
Testing done:

Merge Checklist

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[codecov]

Codecov Report

Merging #582 (a5cf219) into main (2f1be1b) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #582   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          119       119           
  Lines         7656      7676   +20     
  Branches      1700      1707    +7     
=========================================
+ Hits          7656      7676   +20     

Impacted Files	Coverage Δ
src/braket/_sdk/_version.py	100.00% <100.00%> (ø)
src/braket/parametric/free_parameter_expression.py	100.00% <100.00%> (ø)