Add gang-based radio channel restrictions

[AndyBodnar]

Summary

Adds native support for gang-based radio channel restrictions alongside existing job restrictions.

• Players with a matching job (when on duty) OR matching gang can access restricted channels
• Updated config format to support both jobs and gangs arrays
• Works on both client-side and server-side (pma-voice) validation

New Config Format

Config.RestrictedChannels = {
    -- Job-only channel
    [1] = { jobs = { "police", "ambulance" } },

    -- Gang-only channel
    [500] = { gangs = { "ballas", "vagos" } },

    -- Mixed channel (job OR gang can access)
    [999] = { jobs = { "police" }, gangs = { "lostmc" } }
}

Changes

• config.lua: New format with documentation, supports jobs and gangs arrays
• client.lua: Added HasChannelAccess() function to check both job and gang access
• server.lua: Added HasChannelAccess() for pma-voice channel validation

Test plan

Tested with LuaGround (21 tests passed):

• Job-only channels work as before
• Gang-only channels allow matching gangs
• Mixed channels allow job OR gang access
• Unrestricted channels remain open
• Server-side validation works correctly

Test file: https://github.com/AndyBodnar/LuaGround/blob/main/tests/qb_radio_gang_test.lua

Fixes #65