Add an alternative for env.SLACK_WEBHOOK_URL for reusable workflows

[sneetsher]

Is your feature request related to a problem? Please describe.

I was trying to break down long github workflow using reusable workflow. However, act10ns/slack does only support env.SLACK_WEBHOOK_URL as environment variable while reusable workflow is missing that context and has only inputs & secrets.

Describe the solution you'd like

If possible to have an alternative way to set SLACK_WEBHOOK_URL using secrets which kind of common or through direct parameter with:

Describe alternatives you've considered

Currently, no workaround it. I just keep single file for whole process.

Additional context

No