* exec: add a client.users configuration block
For now just add min/max dynamic user values; soon we can also absorb
the "user.denylist" and "user.checked_drivers" options from the
deprecated client.options map.
* give the no-op pool implementation a better name
* use explicit error types to make referencing them cleaner in tests
* use import alias to not shadow package name
* exec2: implement dynamic workload users taskrunner hook
This PR impelements a TR hook for allocating dynamic workload users from
a pool managed by the Nomad client. This adds a new task driver Capability,
DynamicWorkloadUsers - which a task driver must indicate in order to make
use of this feature.
The client config plumbing is coming in a followup PR - in the RFC we
realized having a client.users block would be nice to have, with some
additional unrelated options being moved from the deprecated client.options
config.
* learn to spell
* exec2: implement a dynamic users pool
This PR adds an implementation of a Pool from which dynamic users can
be allocated on behalf of tasks making use of an upcoming feature of
Nomad client (dynamic users).
A task hook and client plumbing, etc. will be in follow up PRs.
* no need for randomness assertion
