Add AdvancedFuzzer for better OSS-Fuzz coverage

[vishalcoc44]

Details

This new fuzzer exercises additional code paths not covered by existing fuzzers:

1. KeyTransformer - Tests custom key transformation during flatten/unflatten
2. Custom Brackets - Tests withLeftAndRightBrackets() with various bracket combinations
3. Reader Constructors - Tests JsonFlattener(JsonCore, Reader) constructors
4. ignoreReservedCharacters() - Tests the option to skip reserved character checking
5. Static Methods - Tests JsonFlattener.flatten(String) and JsonFlattener.flattenAsMap(String)
6. Map-based Unflatten - Tests JsonUnflattener.unflatten(Map) and JsonUnflattener.unflattenAsMap(Map)

Testing

Locally verified with Jazzer:

• AdvancedFuzzer achieves ~5,000 coverage (vs ~600 for existing fuzzers combined)
• All existing fuzzers continue to work