feature: allow to the user to define a balance multiplier for tokens strategies

[lucasmenendez]

For some uses cases could be useful to reduce or increase the importance of a token on a predicate. It will modify the relevance of the balance of every token in the resulting voting weight that will be registered in the censuses of this strategy. The multiplier will be optionally defined in the token information of the strategy, and must be a float number (by default 1.0).

Here is a proposal of new the API request:

{
    "alias": "strategy_alias",
    "predicate": "MON AND (ANT OR USDC)",
    "tokens": {
        "MON": {
            "ID": "0x1234",
            "chainID": 5
        },
        "ANT": {
            "ID": "0x1234",
            "chainID": 1,
            // "minBalance": "1"
            "minBalance": "1",
            "balanceRatio": 0.8
        },
        "USDC": {
            "ID": "0x1234",
            // "chainID": 1
            "chainID": 1,
            "balanceRatio": 1.3
        }
    }
}