[VS Incentives] refactor: unexported addToGaugeRewards function without token transfer

[p0mvn]

Context

AddToGaugeRewards takes in a sender. We use it for distributing incentives from group gauge to underlying gauges.

The problem is that we end up transferring tokens redundantly from module account to itself.

A/C

• introduce addToGaugeRewards that does not take in the receiver and does not transfer tokens
• create tests
• make AddToGaugeRewards use addToGaugeRewards to reduce code duplication
• Update this logic: [VS Incentives]: Update AllocateAcrossGauges to support volume splitting #6431 (comment)