Add email attachment support to the Email action

[b-turchyn]

Adds the ability to attach files to emails. This is useful if a previous step generates a report that you then want to email out.

I extracted out the building of *mail.Message to make the code more testable without needing a fake SMTP server. I opted to test for various headers that we would expect given our input.

Documentation updated to cover the new parameter.

• What kind of change does this PR introduce? feature, docs update
• What is the current behavior? No current support for file attachments
• What is the new behavior (if this is a feature change)? Attachments can now be added to emails sent by the email action
• Does this PR introduce a breaking change? No. Tests were written on the previous message building logic before making any changes.