Support for non-string object keys in SAX handler (MsgPack compatibility) #134

Description

Currently, the SAX API for parsing (JSON/MsgPack) only accepts string keys for objects, following JSON’s specification. However, MessagePack allows keys of any type (e.g., null, integer, double, boolean).
This issue proposes extending the SAX handler to support non-string keys for MsgPack, while ensuring the JSON parser rejects non-string keys (to remain spec-compliant).

Acceptance Criteria

• SAX handler accepts keys of type: null, integer, double, boolean, string for MsgPack.
• SAX handler rejects non-string keys for JSON.