Allow technical committee members to veto external proposals

[luispdm]

• pallet_membership has been added: not only it enables technical committee members to veto, it also adds the ability to add, remove, swap and set members of any collective (council and technical committee in our case). Use this pallet to manage members of a collective
• any type of external proposal can be vetoed (whether super majority approves, against or simple majority)
• once a proposal has been vetoed, it cannot be resubmitted during the cooloff period in the form of public proposal or external proposal with super majority approves. However, during the cooloff period, the same proposal can be resubmitted via super majority against or simple majority voting system
• technical committee members must be set only via the technicalCommitteeMembership pallet, as those changes will propagate to the technicalCommittee collective pallet. The other way around is not true. In other words: if a member is added via the technicalCommittee collective pallet, they will not be part of the technicalCommitteeMembership pallet, meaning that their veto won't be allowed as the origin is violated