Deprecate /deployment-overlay=*/content=*:read-content, replace with a response stream variant

The /deployment-overlay=/content=:read-content op is returning file content as a DMR ModelNode of type STRING. We need to deprecate and eventually drop this operation. Reading these files should use the response attachment mechanism.

Probably we should add a 'stream' attribute to the resource as the replacement. Perhaps other attributes to mirror /subsystem=logging/log-file=*.