It was originally factored out for unit testing, but these days we just
use KargsEditor which has its own tests.  Simplify by using KargsEditor
directly, with a separate initial probe to ensure we can find the
ignition.platform.id.

This has us rewriting the BLS config twice if `--platform` is specified,
but that option is non-performance-critical and also an edge case, so
let's aim for legibility.

Preparatory for next commit.

Once CoreOS stops enabling the serial console by default, users will
need a way to enable it themselves.  This really only affects bare
metal, since other platforms have a consistent console configuration,
so coreos-installer is a good place to handle it.  Users can already
pass console= kargs, but we need a way to specify GRUB commands,
ideally without having to actually use GRUB syntax.

Add a --console option that accepts kernel console= syntax, since
that's already familiar.  Parse the option and convert it to both a
console karg and GRUB commands, using the existing mechanism for GRUB
command substitution.  If --platform is also specified, have --console
override that platform's default console settings.  If --console is
specified more than once, aggregate the console settings, following the
multiple-console semantics for GRUB (use both consoles) and the kernel
(use all consoles, with the last console primary).

There are serial console features only supported by GRUB (stop bits) or
by the kernel (RTS/CTS flow control).  Support the common subset and
reject the rest.  Also, round-trip console kargs to a less-compact but
semantically equivalent format, since explicit is good and it saves
some code complexity.

Document this as "bootloader console" rather than "GRUB console" to
allow for future bootloaders other than GRUB.

If a flag was absent from features.json, we were failing to parse rather
than defaulting the flag to false.  This didn't matter in practice because
both flags were introduced into production at the same time as the file
in FCOS 35.20211215.2.0.

Use the installer-config-directives features.json object introduced by
coreos/coreos-assembler#3083 to check whether
the image's coreos-installer is new enough to support --console.

Rather than manually adding new fixtures to every test individually, have
each test declare its minimum supported fixture, and then automatically
run the test on any newer fixtures.

Fixes failure to invoke some tests on 2022-02 fixture.

In particular, it includes the "console" directive.

If one or more console= kargs are specified to the install or iso/pxe
customize subcommands, and Console can successfully parse all of those
kargs, warn the user that they might benefit from specifying them using
the console mechanism instead.

For legibility, wrap the message at 80 characters, but avoid wrapping in
the middle of the example argument lists.  Use the textwrap crate to do
this.  textwrap is already a clap dependency without default features, and
we also avoid non-default features here, so this doesn't increase the
binary weight.