client: set environment variable indicating set of reserved cpu cores

This PR injects the 'NOMAD_CPU_CORES' environment variable into
tasks that have been allocated reserved cpu cores. The value uses
normal cpuset notation, as found in cpuset.cpu cgroup interface files.

Note this value is not necessiarly the same as the content of the actual
cpuset.cpus interface file, which will also include shared cpu cores when
using cgroups v2. This variable is a workaround for users who used to be
able to read the reserved cgroup cpuset file, but lose the information
about distinct reserved cores when using cgroups v2.

Side discussion in: https://github.com/hashicorp/nomad/issues/12374

We expect every Nomad API client to use a single connection to any
given agent, so take advantage of keep-alive by switching the default
transport to `DefaultPooledClient`. Provide a facility to close idle
connections for testing purposes.

Restores the previously reverted #12409
Co-authored-by: Ben Buzbee <bbuzbee@cloudflare.com>

exec: fix exec handler test

When a service is updated, the service hooks update a number of
internal fields which helps generate the new workload. This also
needs to update the namespace for the service provider. It is
possible for these to be different, and in the case of Nomad and
Consul running OSS, this is to be expected.

This change modifies the template task runner to utilise the
new consul-template which includes Nomad service lookup template
funcs.

In order to provide security and auth to consul-template, we use
a custom HTTP dialer which is passed to consul-template when
setting up the runner. This method follows Vault implementation.
Co-authored-by: Michael Schurter <mschurter@hashicorp.com>

Fixup this test to handle cgroups v2, as well as the :misc: cgroup

This reverts commit 6e1270dd.

The only difference is DefaultTransport sets DisableKeepAlives

This doesn't make much sense to me - every http connection from the
nomad client goes to the same NOMAD_ADDR so it's a great case for keep
alive. Except round robin DNS and anycast perhaps.

Consul does this already
https://github.com/hashicorp/consul/blob/1e47e3c82b6eb937baab0688a67cf7ef334ce42c/api/api.go#L397

disconnected clients: Feature branch merge

* allocrunner: Remove Shutdown call in Reconnect
* Node.UpdateAlloc: Stop orphaned allocs.
* reconciler: Stop failed reconnects.
* Apply feedback from code review. Handle rebase conflict.
* Apply suggestions from code review
Co-authored-by: Tim Gross <tgross@hashicorp.com>

cli: fixup service test delete by using atomic actions.

Co-authored-by: Derek Strickland <1111455+DerekStrickland@users.noreply.github.com>

Copy change, simplifies explanation for no related eval chart

raw_exec: make raw exec driver work with cgroups v2

* TaskGroup: Validate that max_client_disconnect and stop_after_client_disconnect are mutually exclusive.

* structs: Add alloc.Expired & alloc.Reconnected functions. Add Reconnect eval trigger by.

* node_endpoint: Emit new eval for reconnecting unknown allocs.

* filterByTainted: handle 2 phase commit filtering rules.

* reconciler: Append AllocState on disconnect. Logic updates from testing and 2 phase reconnects.

* allocs: Set reconnect timestamp. Destroy if not DesiredStatusRun. Watch for unknown status.

* comments: update some stale comments referencing deprecated config name

* planner: expose ServerMeetsMinimumVersion via Planner interface
* filterByTainted: add flag indicating disconnect support
* allocReconciler: accept and pass disconnect support flag
* tests: update dependent tests