This PR adds support for the raw_exec driver on systems with only cgroups v2.

The raw exec driver is able to use cgroups to manage processes. This happens
only on Linux, when exec_driver is enabled, and the no_cgroups option is not
set. The driver uses the freezer controller to freeze processes of a task,
issue a sigkill, then unfreeze. Previously the implementation assumed cgroups
v1, and now it also supports cgroups v2.

There is a bit of refactoring in this PR, but the fundamental design remains
the same.

Closes #12351 #12348

* renamed: command/debug.go -> command/operator_debug.go
* website: rename debug -> operator debug
* website/pages/api-docs/agent: name in api docs

* reduce memory required for logmon, docker_logger and executor processes

* comment early importing

This commit uses the go-colorable library to enable support for coloured
UI output on Windows. This acts as a compatibility layer that takes
standard unix-y terminal codes and translates them into the requisite
windows calls as required.

* client/executor: refactor client to remove interpolation

* executor: POC libcontainer based executor

* vendor: use hashicorp libcontainer fork

* vendor: add libcontainer/nsenter dep

* executor: updated executor interface to simplify operations

* executor: implement logging pipe

* logmon: new logmon plugin to manage task logs

* driver/executor: use logmon for log management

* executor: fix tests and windows build

* executor: fix logging key names

* executor: fix test failures

* executor: add config field to toggle between using libcontainer and standard executors

* logmon: use discover utility to discover nomad executable

* executor: only call libcontainer-shim on main in linux

* logmon: use seperate path configs for stdout/stderr fifos

* executor: windows fixes

* executor: created reusable pid stats collection utility that can be used in an executor

* executor: update fifo.Open calls

* executor: fix build

* remove executor from...

This PR allows aliased commands to be autocompleted but still filters
them from the help output.

This PR hides non-user CLI commands from the list of autocomplete
suggestions.

This PR adds a version package and consolidates version strings into a
Version struct.

This PR adds command autocompletion to the CLI.

This PR removes our custom handling of the version flag and updates job
history to use a version flag instead of `-job-version`.