WIP fetch command - support reading url from variable

[notryanb]

Currently, when passing a variable, $it, to the fetch command, it breaks with a parse error because it is expecting a Path where it finds a String value. This PR adds support to create a PathBuf from a Primitive::String so values can be read from tables and passed into fetch.

Having this feature is helpful when calling APIs with long query strings.

Example

Example json file, but could be data from anywhere.

{ "url": "https://book.nushell.sh/" }

The following works as if we directly called fetch https://book.nushell.sh
open test_file.json | get url | fetch $it

TODO

• Needs automated testing

[gitpod-io]