enhancement: Add ability to attach files to email

[kaymckay]

PR for go-vela/community#513

adds ability to attach files to an email. This PR corrects the use of email.Attachment and adds EmailFilename to be the file that contains the contents of the email.

[codecov]

Codecov Report

Merging #6 (a5affcb) into main (aaeaa9c) will increase coverage by 2.47%.
The diff coverage is 40.62%.

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
+ Coverage   18.45%   20.93%   +2.47%     
==========================================
  Files           3        3              
  Lines         428      449      +21     
==========================================
+ Hits           79       94      +15     
- Misses        346      353       +7     
+ Partials        3        2       -1     

Impacted Files	Coverage Δ
cmd/vela-email/main.go	0.00% <0.00%> (ø)
cmd/vela-email/plugin.go	53.71% <81.25%> (+5.24%)	⬆️

[wass3r]

couple of comments below.

also,

1. is this a breaking change?
2. we might need attribution for the test image in use depending on where it came from, unless you created it yourself?

[kaymckay]

@wass3r this isn't a breaking change, and i'm not sure what you mean by attribution but i pulled it off the internet. I can replace it with a target image if needed