Add optional environment variables to get workflow from another repo

[marksmithsonuk]

Description

This PR adds the option to set 'custom' environment variables for the GitHub repository and owner.

Motivation

We have a lot of repositories in our organisation, so to avoid having to add the exporter workflow to each of them and maintain them, I was looking at ways to run a single workflow and have all the other repos 'report' to it (via a repository_dispatch API call). In order for this to work though, we need to override the GITHUB_REPOSITORY environment variable, which isn't possible currently in a GitHub Actions workflow. So we needed to be able to set a different environment variable which could be ignored if the workflow was just running in the same repository. I've also added a new environment variable for GitHub owner - although we don't need this ourselves it might be useful for someone.

Example

      - name: Export GHA to New Relic
        id: exporter
        uses: newrelic-experimental/gha-new-relic-exporter@latest
        env:
          GHA_TOKEN: <TOKEN>
          NEW_RELIC_LICENSE_KEY: <KEY>
          GHA_RUN_ID: ${{ github.event.client_payload.workflow_run_id }}
          GHA_RUN_NAME: ${{ github.event.client_payload.workflow_name }}
          GHA_REPOSITORY_OWNER: ${{ github.repository_owner }}
          GHA_REPOSITORY: ${{ github.event.client_payload.repository }}

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