Alias for classic mirrored queue pids field

[michaelklishin]

Classic mirrored queues have originally used an unfortunate term, "slave", to refer to secondary replicas. We have eradicated this from the docs years ago but it is still a field served by the HTTP API and displayed/accepted by CLI tools.

Given a broad range of tools that use both, renaming it would be a significant breaking change. However, we can and should give those who do not want to refer to that field in their own applications, tools or command line usage an option.

For example, we can alias slave_pids as mirror_pids in CLI tools, their underlying internal API responses and HTTP API responses. This won't break any existing tooling but will give the community an option of using a non-controversial field name.

In the long term we expect classic mirrored queues to be deprecated and removed, which would make the field go away for good.