[Feature] Add `group-strategy` option for tcp-brutal

Verification Steps

• I have read the documentation and confirmed that this feature is not implemented
• I have searched for the feature request I want to propose in the Issue Tracker and did not find it
• I am a non-Chinese user.

Description

https://github.com/HyNetworks/tcp-brutal/releases/tag/v2.0.0 tcp-brutal 2.0.0 added a group_id field to TCP_BRUTAL_PARAMS, which makes it possible to apply a single download rate limit to a group of connections sharing the same group_id.

I suggest adding a group-strategy option to brutal, accepting the following values:

• "user" - a group_id is assigned to every incoming connection based on the authenticated user, so all connections belonging to one user share a single download limit.
• "src-ip" - a group_id is assigned to every incoming connection based on its source address.

If the option isn't set or is empty, the behavior stays exactly as it is today: no group_id is sent and the limit applies to each connection independently.

Example:

mux-option:
  brutal:
    enabled: true
    group-strategy: user # or src-ip
    up: 100
    down: 100