Add custom separator overload to Message::toString - #757

Closes #752.

Summary

• Add ABI-preserving Message::toString(char separator) overloads for both return-by-value and caller-provided-buffer usage.
• Preserve the existing overloads and byte-for-byte SOH output.
• Keep BodyLength and CheckSum calculated from the standard SOH wire format, so alternate separators are explicitly display/logging output rather than transmissible FIX messages.
• Replace only field boundaries, preserving embedded SOH bytes in DATA fields and handling nested repeating groups.

Compatibility

No existing signature or symbol is changed. Existing calls to toString() and the custom field-tag overloads retain their current behavior.

Testing

• cmake --build build --parallel 4
• ./lib/ut 'MessageTests' --quickfix-spec-path spec — 221 assertions passed
• ./lib/ut --quickfix-spec-path spec — 1,997 assertions across 53 test cases passed
• Generated and compiled the SWIG 4.2.1 Python wrapper, then runtime smoke-tested Message.toString("|")