Extract reward token approve transaction to a separate function

[Stefan-Ethernal]

Description

Extract approve reward pool transaction as a separate function, to make code cleaner.

Changes include

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• I have assigned this PR to myself
• I have added at least 1 reviewer
• I have added the relevant labels
• I have updated the official documentation
• I have added sufficient documentation in code

Testing

• I have tested this code with the official test suite
• I have tested this code manually

[Nemanja0x]

Some test have failed, probably we should check those before merge.

[Stefan-Ethernal]

Some test have failed, probably we should check those before merge.

You are right, I've made a mistake. We must send approve transaction to the reward token, independently which token is used for rewards (native or custom one). Since approve transaction allows reward pool to spend reward tokens on behalf of reward wallet. This is being done due to the fact that reward pool distributes rewards (see: https://github.com/0xPolygon/core-contracts/blob/dev/contracts/child/validator/RewardPool.sol#L77-L80).

Regarding this PR, functionality wise it is the same as before. However I find it necessary to extract reward token approval to a separate function.