all games: Add ConVars for configuring proximity voice chat

[copperpixel]

Description

Currently, proximity voice chat exists in the engine but it is hard-coded to be disabled. This PR adds ConVars allowing proximity voice chat to be configured by server admins or custom gamemodes. By default it is disabled.

Name	Default value	Description
sv_proximity_voice_enable	0	Enable proximity voice chat
sv_proximity_voice_distance	2048	Max distance at which voice will be projected

proximity_vc_convars.webm

When proximity voice chat is enabled, the proximity_voice tag is appended to server's sv_tags.

[Megawolf7339]

it might be a good idea to add this to the auto tag list in tf2

[copperpixel]

it might be a good idea to add this to the auto tag list in tf2

Implemented