Porting uid/gid mapping ID conversion logic from idtools

[henry118]

I am working on a feature in containerd to support multiple uid/gid mappings containerd/containerd#10307.

My current implementation of the ID conversion logic is based on the idtools code in Moby.

Since the moby/sys package also contains code for ID mapping, such as parsing /etc/{subuid,subuid} and the IDMap type, would it be nice to also include the logic for ID conversions (ie. ToHost()/ToContainer() funcs)? This way containerd can have much cleaner implementation.

WDYT?