Make the payload delimiter configurable

This reverts the default payload delimiter to a period and allows setting the `payload-delimiter` option to override it.

src/slack-send.js

@@ -98,8 +98,9 @@ module.exports = async function slackSend(core) {
       }
 
       if (webhookType === SLACK_WEBHOOK_TYPES.WORKFLOW_TRIGGER) {
-        // flatten JSON payload (no nested attributes), with nested keys delimited with underscores.
-        const flatPayload = flatten(payload, { delimiter: '_' });
+        // flatten JSON payload (no nested attributes).
+        const payloadDelimiter = core.getInput('payload-delimiter') || '.';
+        const flatPayload = flatten(payload, { delimiter: payloadDelimiter });
 
         // workflow builder requires values to be strings
         // iterate over every value and convert it to string