Introduce FS API

[diptanu]

The following APIs has been introduced in the client -

/v1/client/fs/ls/:alloc-id/?path=/path/relative/to/alloc/dir

/v1/client/fs/stat/:alloc-id/?path=/path/relative/to/alloc/dir

/v1/client/fs/readat/:alloc-id?path=/path/relative/to/alloc/dir&limit=K&offset=N

@armon These APIs currently don't do any forwarding because to forward the request, the Agent has to make two API calls to the server, first get the allocation for the alloc id and then another API call to get the node information from the node id of the allocation. So every API call is going to result into two API calls to the server.
Instead, the tools using the APIs could make the two calls to the server to determine the correct addr of the client on which the allocation is running and cache the information. And from there on keep making the API calls to the same client for the allocation. What you think? (It's easy to introduce forwarding if we want)

[dadgar]

I would add an interfacet:

type AllocDirFS interface {
    List(path string) ([]*allocdir.AllocFileInfo, error)
    Stat(path string) (*allocdir.AllocFileInfo, error)
    ReadAt(path string, offset int64, limit int64) (io.Reader, error)
}

[dadgar]

at a

[github-actions]

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