cli/namespace: add detailed flag to namespace list

[jasonodonnell]

This enhances the namespace list command to include a detailed flag which will output additional data such as namespace IDs. This is very useful because without this you would either need to use curl or do a vault namespace lookup for every namespace you wanted the ID for in a loop. This lets you get all the information in one command:

$ vault namespace list -detailed
Keys    custom_metadata    id       path
----    ---------------    --       ----
bar/    map[]              yva5e    bar/
foo/    map[]              iZULg    foo/

$ vault namespace list -detailed -format=json
{
  "bar/": {
    "custom_metadata": {},
    "id": "yva5e",
    "path": "bar/"
  },
  "foo/": {
    "custom_metadata": {},
    "id": "iZULg",
    "path": "foo/"
  }
}