Parse the sort string and return a slice of SortSpec.

ProcessDevicesTable will apply sorting (if required), slicing (if required) and the massage/transformation of data to produce a final `*DevicesTable` that has everything required to render.

These two buckets contain FULL data.

AllHeadersList must contain the complete list of headers.

AllHeadersMap initializes a map of HeaderMatchName to Header.

CtxKeyConfig holds all config settings that were resolved from the environment/config file/cli flags.

DefaultColumnSet is the column set that ships out of the box.

DevicesView has everything needed to be rendered.

TailnetView has everything known about a Tailnet.

WrappedDevice is a type that wraps the core `tailscale.Device` type.