Add `MinContributors` to avoid malicious campaigns

[balqaasem]

One person could buy all the funds allocated to a campaign, this is not good for the community and the bootstrapping of the token on SetSwap DEX, therefore we must have a minimum number of contributors before a project is considered successful.
This parameter MinContributors is then used relative to the goal of the campaign to determine the max_contribution per contributor.
Formula: max_contribution = goal / MinContributors

For example, if we say that the MinContributors is 200 and a Campaign-A has a goal: Balance = 200_000, then the max_contribution = goal / MinContributors which in this example is 200_000 / 200 = 1_000, therefore each contributor is limited to contributing 1_000.