Add define/ifdef blocks, alternate sprintf implementation via FLATBUFFERS_PREFER_PRINTF [C++] (#4700)

* Add define/ifdef blocks for FLATBUFFERS_PREFER_PRINTF to avoid using std::*streams for idl_parser

* Use string::size() as limit in snprintf

* Refactored FLATBUFFERS_PREFER_PRINTF guarded feature into NumToStringImplWrapper around sprintf

* Remove '.0' where not needed from IntToDigitCount

* Remove leading dot from name in GetFullyQualifiedName when FLATBUFFERS_PREFER_PRINTF is enabled

* Return string directly from conversion functions where possible when FLATBUFFERS_PREFER_PRINTF is enabled

* Use string instead of stringstream for GetFullyQualifiedName

* Revert removing leading dot from GetFullyQualifiedName, it does need to be there for parity with the stringstream implementation

* Dot is single char in Namespace::GetFullyQualifiedName

* Remove trailing (duplicate) null-byte from NumToStringImplWrapper when using FLATBUFFERS_PREFER_PRINTF.

* Update preprocessor indenting (and use clang-format off/on) for FLATBUFFERS_PREFER_PRINTF

* Reduce whitespace, unneeded braces in FLATBUFFERS_PREFER_PRINTF string width functions

* Remove unneeded use of iostream from idl_parser.cpp, std::string is used instead

* Tell snprintf to use the trailing null byte expected at the end of std::string buffer