Add bindable custom VScript input actions

[ywgATustcbbs]

Summary

• add four bindable client commands (rd_custom_input_0 through rd_custom_input_3) with validated, rate-limited server dispatch
• expose OnCustomInput(cmd_index, player) to VScript listener scopes
• use custom input 0 to open the Traitors challenge menu while preserving the existing chat-command path
• add the four actions to legacy and modern keyboard settings, with custom input 0 bound to O by default

Validation

• Client Debug and Release builds
• Server Debug and Release builds
• in-game verification of the Traitors menu callback
• modern Controls resource verified with -override_vpk
• git diff --check

[ywgATustcbbs]

This pr is fully AI generated, with human code review line by line and in-game test.

It is intended to improve the Traitors challenge skill menu UX. (many players were confused and dont know how to open the skill menu.)

And, for other modders, they may use these custom key binddings to make rpg-like skill system.

[jhh8]

does UserConsoleCommand function lack something?

[ywgATustcbbs]

never know there is such a concmd. will close and cteate a new pr