This adds an initial implementation of ACLs for HostVolumes.
Because HostVolumes are a cluster-wide resource, they cannot be tied to
a namespace, thus here we allow similar wildcard definitions based on
their names, tied to a set of capabilities.
Initially, the only available capabilities are deny, or mount. These
may be extended in the future to allow read-fs, mount-readonly and
similar capabilities.
This adds `alloc-exec` capability to allow operator to execute command into a
running task. Furthermore, it adds `alloc-node-exec` capability, required when
the alloc task is raw_exec or a driver with no FSIsolation.
This capability will gate access to features that allow interacting with
a running allocation, for example, signalling, stopping, and rescheduling
specific allocations.
