Created BASIC rule implementation

[Bullrich]

Basic rule

Implemented the functionality of the basic rule.

Because every rule ends up being simplified into:

{
  min_approvals: number;
  teams?: string[];
  users?: string[];
}

I created a basic logic that evaluates that. After this, we can use the result of those smaller conditions to evaluate the more complex rules.

I added a lot of tests and tried to do as many comments to explain the logic as possible.

Teams API

Created class which handles the teams token and obtains the team members of a team.

This class is small but handles the token used for such authentication and separates the concern.

If we decide to replace GitHub teams for on-chain data (like mentioned in paritytech/opstooling#245) this would let us to simply switch the implementation with minimal alterations.

Closes #22 and closes #9