argparse with option/value like --gpg-options "--homedir=/home/user" errors out but adding binding op works.

Bug report

Bug description:

--gpg-options is defined as follows:

    gpg_options=dict(
        metavar=_("options"),
        action=SplitOptionsAction,
        help="Verbatim gpg options.  May be supplied multiple times.",
        default=dflt(config.gpg_options),
    ),

This does not work (using space):

prog --gpg-options "--homedir=/home/user"

and gets argument --gpg-options: expected one argument

But this one does (using the binding op):

prog --gpg-options="--homedir=/home/user"

with gpg_options being set correctly.

The deprecated optparse handled this correctly.

See Also:
https://gitlab.com/duplicity/duplicity/-/issues/795
https://gitlab.com/duplicity/duplicity/-/issues/816

CPython versions tested on:

3.8, 3.9, 3.10, 3.11, 3.12

Operating systems tested on:

Linux, macOS

Start with the argparse handling for options whose values begin with --, using the reported --gpg-options "--homedir=/home/user" command as the reproducer. Add coverage for the space-separated form alongside the binding-operator form, and confirm the option value is accepted correctly across the relevant tests.