Adds option to copy packer output to file

[danshaub]

Before, there was no way to capture the output generated by the packer command, and as such, there was no way to upload the output as an artifact for later use or for logging purposes. This pull request simply adds the output_file option without changing any other functionality.

This change is reflected here: https://github.com/marketplace/actions/packer-github-actions-dh_test

[hashicorp-cla]

All committers have signed the CLA.

[JenGoldstrich]

Hi @danshaub Thank you for the proposed changes - this makes sense logically and sorry for the delay in getting back to you but @ksatirli and I are about to release changes (under the main branch) which change this action to be a more direct interface to the packer binary which will enable this change.

Closing this PR for now, the old action will still be available under the master branch