Slack template file fixer action

1. Description

This action is a workaround for this open issue in the slack-github-action, specifically when expanding context related variables in a template file.

1.1 Inputs

Name	Description	Optional
payload-file-path	Path to JSON-formatted template file to fix	✔️ *
payload	JSON string template file to fix	✔️ *
replacements	Ad-hoc replacements to perform on the payload-file-path; see Ad-hoc replacements	✔️
channel-id	Slack channel to send the message to	❌
slack-bot-token	Access token to the Slack API	❌

📝 Note:

Either payload or payload-file-path must be provided, the step will fail if none is provided.

1.2 Ad-hoc replacements

Given the following workflow step:

    name: Send notifications
    uses: rapyuta-robotics/[email protected]
    with:
      payload-file-path: ./failure_notification.json
      replacements: |
        NOTIFICATION_HEADER=":warning: Failed ${{ github.sha }}"
        NOTIFICATION_FOOTER="See <www.example.com>"
        WRONG=This will not be replaced
      channel-id: ABCDFG
      slack-bot-token: ${{ secrets.SLACK_NOTIFICATIONS_APP_TOKEN }}

📝 Note:

The input replacements may use any valid github action expression and they will be expanded to its corresponding values before running the replacement step; in this example, ${{ github.sha }} will be replaced by the commit sha before running the actual replacement.

This action will perfom the following steps:

1. Take every new line under ad-hoc-replacements as a potential replacement in the template, in this case there will be 3 potential replacements.
2. Validate each potential candidate according to /(.+?)="(.+?)"/, if it doesn't match the regex, it will be skipped; in this case WRONG=This will not be replaced will be discarded as it is missing the quotation marks.
3. Every ocurrence of the first capture group preceeded by a percentage sign present in the template will be repaced by the second capture group, that is, every ocurrence of %NOTIFICATION_HEADER in the template will be replaced by :warning: Failed 1a6184908f91b86ba226f4436a7b0323fbc2820b, and likewise ocurrences of %NOTIFICATION_FOOTER will be replaced by See <www.example.com>.