Add support for custom email headers via JSON input (#264)

* Adding custom header with JSON support

* Adding headers in test workflow

---------

Co-authored-by: W506810_wexinc <spiros.kalogeropoulos@wexinc.com>

main.js

@@ -115,6 +115,7 @@ async function main() {
             required: false,
         });
         const envelopeTo = core.getInput("envelope_to", { required: false });
+        const headers = core.getInput("headers", { required: false });
 
         // if neither to, cc or bcc is provided, throw error
         if (!to && !cc && !bcc) {
@@ -166,6 +167,7 @@ async function main() {
                         ? getText(htmlBody, convertMarkdown)
                         : undefined,
                     priority: priority ? priority : undefined,
+                    headers: headers ? JSON.parse(headers) : undefined,
                     attachments: attachments
                         ? await getAttachments(attachments)
                         : undefined,