Add --service for running as a windows service for renewal and rekeying

[unreality]

Name of feature:

Adds a new command line flag --service which will run as a Windows Service. Users can install the service by running sc.exe create step-renew binPath= "path_to_step_cli.exe ca renew --service --ca-url=the_ca_url --root=path_to_root_ca.crt certfile.crt keyfile.key as an Administrator.

Pain or issue this feature alleviates:

Allows renewal via windows service as an alternative to --daemon on windows hosts

Why is this important to the project (if not answered above):

Is there documentation on how to use this feature? If so, where?

CLI usage examples have been added

In what environments or workflows is this feature supported?

Windows hosts

In what environments or workflows is this feature explicitly NOT supported (if any)?

Non-windows hosts (use --daemon instead)

Supporting links/other PRs/issues:

💔Thank you!

[maraino]

I suppose we can also add OS custom flags of files that use go tags.

#go:build windows

package ca

import "..."

func rekeyExtraFlags() []cli.Flag {
    return []cli.Flag
        cli.BoolFlag{Name: "service", Usage: "..."},
    }
}

The problem with adding this is that we cannot keep the order, but those flags won't appear on other OSs.

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