Let job args return plugins like they can for hooks/middleware

[brandur]

Here, implement a job args jobArgsWithPlugins interface that lets job
args return a set of Plugins. This follows up the addition of plugins
in #1284, and makes the functionality symmetrical to what's available
with hooks/middleware. I would have done this before, but I forgot that
you could do this until I was looking at docs today. As in #1284 for
client config, Hooks and Middleware continue to be supported on job
args and have not yet been deprecated.

type EmailArgs struct {
    To string `json:"to"`
}

func (EmailArgs) Kind() string { return "email" }

func (EmailArgs) Plugins() []rivertype.Plugin {
    return []rivertype.Plugin{
        &EmailPlugin{},
    }
}

type EmailPlugin struct {
    river.PluginDefaults
}

func (p *EmailPlugin) WorkBegin(ctx context.Context, job *rivertype.JobRow) error {
    return nil
}

I also had Codex look for opportunities for implementation clean up
since all of this was feeling like a few too many LOCs. It was able to
clean up ~85 LOCs net, with the main change being to get rid of
PluginLookupInterface + emptyPluginLookup. This removes a layer of
indirection, which is good, but also emptyPluginLookup had become less
useful due to River installing default middleware (ResumableMiddleware)
on all clients whether any has been specified by a client or not.