Let job args return plugins like they can for hooks/middleware - #1337

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.