API: Implement RG_CBasePlayer_HandleSignals hook

[JohanCorn]

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Expose the new ReGameDLL CBasePlayer::HandleSignals hookchain to plugins.

Purpose

Exposes the CBasePlayer_HandleSignals hookchain added in rehlds/ReGameDLL_CS#1195 as RG_CBasePlayer_HandleSignals, so AMXX plugins can run code before or after the zone signal update (buy, bomb, rescue, escape and VIP safety zones), or skip it with HC_SUPERCEDE.

Supersedes #379, which was closed when my fork was deleted.

Approach

Adds RG_CBasePlayer_HandleSignals and its callback, using the updated ReGameDLL API header.

Open Questions and Pre-Merge TODOs

• API: Implement CBasePlayer::HandleSignals hook ReGameDLL_CS#1195 is merged (this PR uses the header from it).
• Built on Windows (MSVC v143, Release, Win32) together with the ReGameDLL change.
• Tested on a dedicated server with a plugin:
  • the pre and post hooks are called for the player
  • HC_SUPERCEDE skips the original function and leaves the player's signal state unchanged, the post hook still runs
  • the original runs again with HC_CONTINUE, and disabling and enabling the hook works
• Green CI run on this PR.

Learning

• Normally called at most once per 0.5 s for each player.
• Earlier attempt: Implement CBasePlayer::HandleSignals Hook #379