Fix arbitrary dll load via json injection

[apsonawane]

This pull request enhances the security of JSON handling in src/config.cpp by introducing a robust JSON string escaping function. This prevents potential JSON injection vulnerabilities when user-supplied values are embedded in JSON documents. The most important changes are:

Security Improvements:

• Added a new EscapeJsonString function to safely escape all necessary characters in strings before embedding them in JSON, handling quotes, backslashes, control characters, and other non-printable characters.
• Updated the SetProviderOption function to use EscapeJsonString on provider_name, option_name, and option_value before inserting them into the JSON document, preventing malicious input from altering the JSON structure.

Dependency Updates:

• Included the <cstdio> header to support the use of std::snprintf in the new escaping function.

[Copilot]

Pull request overview

This PR aims to harden src/config.cpp against JSON injection by introducing EscapeJsonString and using it when building a JSON snippet inside SetProviderOption before parsing it with the repo’s JSON parser.

Changes:

• Added EscapeJsonString(std::string_view) to escape caller-supplied fragments before embedding them into a JSON string literal.
• Updated SetProviderOption to escape provider_name, option_name, and option_value before concatenating the JSON document.
• Added <cstdio> for std::snprintf.