Add functions to manage Acl roles

[mooori]

Note: To be merged after #8. Once #8 is merged I'll rebase this PR, until then the diff here on github is messy: Only the two most recent commits will remain in this PR, previous commits belong to #8.

Overview

• Trait methods are added to AccessControllable:
  • acl_grant_role
  • acl_revoke_role
  • acl_renounce_role
  • acl_revoke_role_unchecked
• The attribute #[access_controllable] adds implementations of these methods.
• The new methods are tested in near-plugins/tests/access_controllable.rs

# Command to execute the tests:
cargo test --test access_controllable

# This makes `workspaces` write more than 1G to /tmp (on my machine).
# Depending on the size of your /tmp, a cleanup might be required afterwards.
du --summarize --total --human-readable /tmp/sandbox-*
rm -r /tmp/sandbox-*

[mooori]

Converting to draft since it'll need to be updated after deciding how to handle *_unchecked methods in #8.