Add toggleable GM powers command

[joshlit]

Summary

• add a GM-only /powers command with toggles for god mode, outgoing damage boost, and attackability
• preserve incoming damage messages while god mode prevents health loss
• multiply outgoing normal and critical damage by 10.0 by default, with optional custom multipliers
• apply the outgoing multiplier centrally in GameLiving.TakeDamage without changing player Effectiveness
• allow hostile mobs and enemy players through the existing GM attack protections when attackable mode is enabled
• add status, explicit on/off controls, help, and reset commands
• group session-only state behind the abstract GMPowers contract exposed through player.GMPowers
• use separate PlayerGMPowers and GMPowersFacade implementations so the shared null object has only false/default getters and no-op setters

Why

Staff need a convenient, session-scoped way to test combat interactions without manually changing account state or multiple unrelated settings. GamePlayer gains only one nullable backing field; its GMPowers facade safely returns the stateless null-object implementation until a power is enabled. Damage boost uses dedicated power state rather than Effectiveness, so resurrection illness, weapon skill, concentration, buffs, and other effectiveness-based systems remain untouched.

Impact

The powers are available only to GM-level accounts and are not persisted across sessions. A real per-player state object is created only when needed, and the backing field returns to null when no powers remain. Existing GM protection remains unchanged unless /powers attackable is explicitly enabled.

Validation

• based on the latest OpenDAoC/master
• dotnet restore GameServer/GameServer.csproj
• dotnet build GameServer/GameServer.csproj --no-restore --nologo -v:minimal
• build succeeded with 0 errors; existing project warnings remain