Add support for build arg files in yaml format.

[featheredtoast]

Adds key/value yaml files as possible input for build arg file. This allows for multiline args support in from build arg files.

Since build arg file splits on newlines, it's currently not possible to do so with build arg file option: Passing yaml would allow us to set the value of build args to any string, including newlines, making this possible:

my-input/myvars.yaml

  foo: |
    first line
    second line
    third line

- task my-oci-build-task
  params:
    BUILD_ARGS_FILE: my-input/myvars.yaml
  platform: linux
  image_resource:
    type: registry-image
    source:
      repository: concourse/oci-build-task
  inputs:
    - name: my-input

setting the build arg of foo to:

first line
second line
third line

closes #122