Add support for reading GOTO functions from JSON

[tautschnig]

Note to reviewers: please review commit-by-commit.

Adds a parser for JSON-encoded goto_functionst and support to symtab2gb to optionally read such from a file and store them in goto_functions of a goto_modelt.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

❌ Patch coverage is 88.00461% with 104 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.43%. Comparing base (b0156a3) to head (6d3ce5d).
⚠️ Report is 7 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/json-symtab-language/json_goto_function.cpp	71.97%	51 Missing ⚠️
src/json-symtab-language/json_symbol.cpp	45.16%	34 Missing ⚠️
src/symtab2gb/symtab2gb_parse_options.cpp	31.81%	15 Missing ⚠️
src/json-symtab-language/json_goto_functions.cpp	92.85%	2 Missing ⚠️
src/util/json_irep.cpp	91.30%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8713      +/-   ##
===========================================
+ Coverage    80.39%   80.43%   +0.03%     
===========================================
  Files         1688     1695       +7     
  Lines       207435   208285     +850     
  Branches        73       73              
===========================================
+ Hits        166776   167524     +748     
- Misses       40659    40761     +102     

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