Add string function to StateCommitment type, use in place of marshalJSON

[Kay-Zee]

Closes #7193

while building the block model for the rest api, we shouldn't manually call marshalJSON, as that will actually double encode this particular value. Instead, just add string function to the type and call that instead

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 41.36%. Comparing base (02b0c42) to head (20b5df2).

Files with missing lines	Patch %	Lines
model/flow/ledger.go	0.00%	3 Missing ⚠️
engine/access/rest/common/models/block.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7194      +/-   ##
==========================================
- Coverage   41.38%   41.36%   -0.02%     
==========================================
  Files        2174     2174              
  Lines      190130   190126       -4     
==========================================
- Hits        78691    78654      -37     
- Misses     104863   104890      +27     
- Partials     6576     6582       +6     

Flag	Coverage Δ
unittests	41.36% <0.00%> (-0.02%)	⬇️

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