Add no copy api variants to json interface

[DmitriyMusatkin]

Issue #, if available:

Description of changes:
There are several json apis that currently take key by cursor, including some heavily used apis like get_object or has_object. Since underlying lib relies on const char, that means that every one of those apis needs to make a temp copy of the key. In grand scheme of things copies are cheap, but for heavy users of json (ex. endpoint resolution) cost ends up quickly.

This PR adds const char variants for all functions taking a key to avoid a copy. And as a side bonus you can now just write get_object(json, "field_name"), which is cleaner in my opinion than having to wrap it in cursor.

Can we just make cursor version not do a copy? Not without major surgery of cJson, which internally relies on c strings a lot.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.23529% with 4 lines in your changes missing coverage. Please review.

Project coverage is 83.82%. Comparing base (c9ead75) to head (a86e8a9).

Files	Patch %	Lines
source/json.c	88.23%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1138      +/-   ##
==========================================
+ Coverage   83.38%   83.82%   +0.44%     
==========================================
  Files          57       57              
  Lines        5994     5991       -3     
==========================================
+ Hits         4998     5022      +24     
+ Misses        996      969      -27     

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