feat: add ability to make all rows in a CSV payload available to each VU

[hassy]

Setting the "name" property on a CSV payload will make all rows available to the VU, under the variable with that name. The type of the value is an array of objects, where each object has properties corresponding to the "fields" spec of the CSV payload.

Example:

config:
  target: https://www.artillery.io
  payload:
    # Load all rows from the CSV file into a "data" variable. Each element of the
    # array will have an "url" and "code" property:
    - path: ./request-response.csv
      name: data
      fields: [url, code]
  plugins:
    expect: {}
scenarios:
  - flow:
    # Loop over each element in the data variable, with loop-over
    # Each element in the "data" array can be accessed via the special
    # $loopElement variable.
    # https://artillery.io/docs/guides/guides/http-reference.html#Looping-through-an-array
    - loop:
        - get:
            url: "{{ $loopElement.url }}"
            followRedirect: false
            expect:
              statusCode: "{{ $loopElement.code }}"
      over: "data"