Simplified Environment Variable Export for Custom Templates

[mlachkar-da]

Describe Request:

Currently, when using custom templates in the CircleCI Slack Orb, users need to manually handle the export of environment variables to customize the template fields. This process can be cumbersome and error-prone.
Maybe we could introduce an environment key under the slack/notify job where users can define environment variables directly within the CircleCI configuration. These variables would then be automatically available within the custom Slack templates, reducing the need for manual export steps.

Examples:

- slack/notify:
    channel: some-channel
    branch_pattern: main
    template: basic_fail_2
    environment:
      FAILURE_MESSAGE: "Please take care of this issue ${SELECTED_VOLUNTEER}!"

Thanks!