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

* Adding custom header with JSON support

* Adding headers in test workflow

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

.github/workflows/test.yml

@@ -46,6 +46,7 @@ jobs:
           convert_markdown: true
           attachments: package.json,action.yml
           priority: high
+          headers: '{"X-My-Test-Header": "Passed"}'
 
       - name: Get mail
         run: |