Partially update the policy of a DAO by introducing smaller proposal kinds

[ctindogaru]

Fixes https://github.com/near-daos/sputnik-dao-contract/issues/59.

Problem: When the policy is being too large, using ChangePolicy proposal type is no longer feasible as the payload is too big and the request fails.

Solution: Introduce 4 new proposal types to allow partially changing the policy:

• ChangePolicyAddOrUpdateRole - used to add a new role to the policy of the DAO. If the role already exists, update it.
• ChangePolicyRemoveRole - used to remove a role from the policy of the DAO.
• ChangePolicyUpdateDefaultVotePolicy - used to update the default vote policy from the policy of the DAO.
• ChangePolicyUpdateParameters - used to update the parameters from the policy of the DAO. Parameters include: proposal bond, proposal period, bounty bond, bounty forgiveness period.