'Path' type for commands lib Arguments

I've wanted a `PathArg` type for the commands libs that will validate that the content its passed is a valid ipfs path, and allow typed access to that to the consumers of the `cmds.Request`

Start by locating the commands library's argument types and the cmds.Request consumers mentioned in the issue. Review existing argument validation and typed access patterns, then define the completion criteria around accepting a PathArg, rejecting invalid IPFS paths, and exposing typed access to consumers.

Written by the indexing model from the issue text.