Remove pending membership storage items and request and revoke membership calls

As discussed in the forum topic: https://forum.duniter.org/t/a-quoi-servent-les-pending-membership/11799, I would like to:

• remove request_membership call
• remove revoke_membership call
• remove PendingMembership storage item
• remove PendingMembershipsExpireOn storage item
• remove smith instance of membership pallet
• add add_member call in authority member pallet symmetrical to remove_member
• allow smith identities to change identity owner key
• make change_owner_key function of authority members pallet available as a call
• remove validate_identity call from identity pallet which is redundant with claim_membership of membership pallet

This would simplify a lot the protocol and remove potential bugs like the one targeted in #155 (closed) while keeping a good separation of concerns and make the event history easier to manage for the indexers.

Also linked to #136 (closed) in a way.

changed milestone to %Horizon

added P5-sometimesoon S-needdiscussion labels

assigned to @HugoTrentesaux

changed milestone to %runtime-800

An alternative suggestion would be to completely remove membership pallet and move the membership expiration block to identity pallet.

removed S-needdiscussion label

changed the description

I'm keeping the smith instance of membership pallet and revoke_membership call for the moment. Will consider this in a second pass.

marked the checklist item remove request_membership call as completed

marked the checklist item remove PendingMembership storage item as completed

marked the checklist item remove PendingMembershipsExpireOn storage item as completed

marked the checklist item remove validate_identity call from identity pallet which is redundant with claim_membership of membership pallet as completed

mentioned in merge request !215 (merged)

closed with merge request !215 (merged)