fix(smtp): add date header and make enums public

[piksel]

• Add missing required Date: header
• Make AuthTypes and EncMethods enum helpers public

[codecov]

Codecov Report

Merging #118 (9e28a0c) into main (1290cec) will increase coverage by 3.84%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##             main     #118      +/-   ##
==========================================
+ Coverage   52.79%   56.64%   +3.84%     
==========================================
  Files          55       55              
  Lines        1161     1167       +6     
==========================================
+ Hits          613      661      +48     
+ Misses        512      462      -50     
- Partials       36       44       +8     

Impacted Files	Coverage Δ
pkg/services/smtp/smtp_authtype.go	0.00% <0.00%> (ø)
pkg/services/smtp/smtp_encmethod.go	62.50% <33.33%> (+6.94%)	⬆️
pkg/services/smtp/smtp.go	81.42% <90.00%> (+0.13%)	⬆️
pkg/services/smtp/smtp_config.go	87.09% <100.00%> (ø)
pkg/services/slack/slack.go	63.63% <0.00%> (+41.41%)	⬆️
pkg/services/gotify/gotify.go	73.33% <0.00%> (+44.76%)	⬆️
pkg/services/gotify/gotify_config.go	86.66% <0.00%> (+86.66%)	⬆️
pkg/services/slack/slack_json.go	100.00% <0.00%> (+100.00%)	⬆️

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