feat: add country code to GXS group metadata

[AniruthKarthik]

This PR adds a mCountryCode field to the GXS group metadata. This is the core library implementation required to support country flags in Channels, Forums, and Posted boards.

Changes:

• Version Increment: Incremented RS_GXS_GRP_META_DATA_CURRENT_API_VERSION to 0xaf02 (Version 3) to accommodate the new field.
• Metadata Expansion: Added mCountryCode (string) to both RsGxsGrpMetaData (internal) and RsGroupMetaData (serializable).
• Serialization Logic:
  • Updated rsgxsdata.cc to handle serialization/deserialization for the new Version 3.
  • Maintained backward compatibility by ensuring Version 1 and Version 2 metadata can still be read (mapping to an empty country code).
• Data Mapping: Updated the assignment operator and rsgxsitems.cc to ensure the country code is correctly propagated between internal GXS structures and
  the serializable items.

Related GUI PR:
This PR provides the backend for: RetroShare/RetroShare#3162

[defnax]

Hi we need in this feature best in RsGxsForumGroup, not in metadata

[AniruthKarthik]

@defnax kindly review