add ServiceBuilder::boxed_clone_sync helper

[jlizen]

As requested in: #777

Adding the ServiceBuilder::boxed_clone_sync() method to match boxed_clone and boxed().

Note that in this case, I used the named, BoxCloneSyncServiceLayer, rather than the layer fn returned by BoxedCloneSyncService::layer(). I don't really see a good reason to prefer the latter. I think the reason that other helpers use that, is historical (named layer didn't exist yet).

Probably we should migrate everything over, but that seems likely to be a breaking change so I held off. Glad to cut an issue on that if you concur.

The only testing was via a doctest. This matches the other helper.

[jlizen]

This is a quickie if you have a minute @seanmonstar , you have context on this from #777