[ADD] ops/server.user: allow to add user to more secondary groups

[Pirols]

It is often necessary to add a user to a secondary group, without dropping it from other secondary groups it already belonged to. usermod provides a -a flag dedicated to this very need. This commit allows to leverage that flag via pyinfra.

In fact, in pyinfra this is even more necessary as facts are collected and never updated, making so that:

from pyinfra import host
from pyinfra.facts.server import Users
from pyinfra.operations import server

server.group(
    name="create main group",
    group="user",
)

server.group(
    name="create secondary group 1",
    group="extra_group1",
)

server.user(
    name="create user `user`",
    user="user",
    groups=["extra_group1"],
)

server.group(
    name="create secondary group 2",
    group="extra_group2",
)

server.user(
    name="Add user to extra group 2",
    user="user",
    groups=[
        *host.get_fact(Users).get("user", {}).get("groups", []),
        "extra_group2",
    ],
)

• yields usermod -G extra_group1,extra_group2 user if user was member of extra_group1 before the task started, or
• yields usermod -G extra_group2 user, if it was not part of the task

Notice that it does not matter whether the user user was added to the extra_group1 group by pyinfra.

[Pirols]

The CI linting seems to be failing, while scripts/dev-lint.sh does not highlight any issue. And the error message in the CI message is cryptic to me.

[Fizzadar]

Nice, thank you for adding this @Pirols!

Re:CI - it's the black formatter failing on a completely unrelated file, so ignoring!