Add cvars to toggle (show/hide) chat messages from some plugins

[altair-sossai]

Several plugins send recurring chat messages that explain game mechanics which more experienced players are already very used to seeing, which ends up causing a bit of chat spam during matches. The goal of this PR is to let server owners turn these messages off through new cvars to avoid that spam. Every cvar defaults to 1 (show), so behavior is unchanged unless an owner explicitly sets it to 0.

1. checkpoint-rage-control.sp

• CVar: crc_announce (default 1)
• Messages disabled when 0:
  • Tank will still lose frustration while survivors are in saferoom.
  • Tank will not lose frustration while survivors are in saferoom.

2. l4d2_slowdown_control.sp

• CVar: l4d2_slowdown_announce (default 1)
• Messages disabled when 0:
  • Water Slowdown has been reduced while Tank is in play.
  • Water Slowdown has been restored to normal.

3. l4d2_tankrage.sp

• CVar: l4d2_tankrage_announce (default 1)
• Message disabled when 0:
  • For every X% Survivors run back, the Tank will have their frustration frozen for Y seconds.

4. l4d_tank_rush.sp

• CVar: l4d_no_tank_rush_announce (default 1)
• Messages disabled when 0:
  • Tank has spawned. Freezing distance points!
  • Tank is dead. Unfreezing distance points!
  • Survivors made it to the saferoom. Unfreezing distance points!
• The existing spawn-sound cvar (l4d_no_tank_rush_spawn_sound) stays independent of the message toggle.

5. l4d2_tank_attack_control.sp

• CVar: l4d2_tank_rock_selector_announce (default 1)
• Messages disabled when 0: the "Tank Rock Selector" control tutorial shown to the Tank player on spawn —
  • [Tank Rock Selector]
  • Reload/!overhand = 2 Handed Overhand
  • Use/!underhand = Underhand
  • M2/!overonehand = 1 Handed Overhand

6. spechud.sp (Hyper-V HUD Manager)

Two independent cvars (the hints target different audiences — the Tank player vs. spectators):

• CVar: spechud_tank_usage_hint (default 1)
  • Message disabled when 0: <HUD> Type !tankhud into chat to toggle the Tank HUD
• CVar: spechud_spectator_usage_hint (default 1)
  • Message disabled when 0: <HUD> Type !spechud into chat to toggle the Spectator HUD