Add functions acl_get_{admins, grantees}

[mooori]

Note To be merged after #8 and #9. Once they are merged, I'll rebase this PR. Until then, the diff here on github is messy (hence still in draft mode). After the rebase, only the most recent commit should remain in this PR.

Overview

Adds methods to retrieve the admins and grantees of roles.

• Trait methods are added to AccessControllable:
  • acl_get_admins
  • acl_get_grantees
• 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-*