Remote GSM

[efecarranza]

Changelog

Add RemoteGSM Implementation
Abstract methods on GSM

[miguelmtzinf]

is this check actually needed? same in restore

• when removing the entity, we check used and limit is 0.
• when adding an entity, it can start with 0 limit.
• use check limit, so only existent entities with non-zero limit are allowed
• restore check used, so only existent entities with non-zero used are allowed

[miguelmtzinf]

apart from this, we are doing a similar check in setLimit with different error. We should be consistent

  function setLimit(address entity, uint256 limit) external onlyOwner {
    require(_entities.contains(entity), 'ENTITY_DOES_NOT_EXIST');