feat(libc): add netdb core API support

[MDLZCOOL]

• Added LIBC_NETDB_CORE config option to enable minimal netdb core API
• Updated netdb.h header file to conditionally compile related functions based on configuration

Summary

Introduced a minimal netdb core API controlled by the new LIBC_NETDB_CORE config option.
This allows enabling essential netdb functions without requiring the full netdb implementation.

Impact

• Existing builds are not affected unless LIBC_NETDB_CORE is explicitly enabled.
• Ensures conditional compilation to avoid unnecessary code inclusion.

Testing

• Verified successful build with LIBC_NETDB_CORE enabled and disabled.
• Checked that related functions in netdb.h are correctly included/excluded based on the configuration.

[MDLZCOOL]

This can fix PX4/PX4-Autopilot#25540 and PX4/PX4-Autopilot#24273.