Implemented DSNs as described in RFC 1891

[wneessen]

• Implemented MAIL FROM RET parameter
• Implemented RCPT TO NOTIFY parameter

Not implemented so far (not sure if we need this at all):

• MAIL FROM ENVID parameter
• RCPT TO ORCPT parameter

TBD:

• Test coverage

[codecov-commenter]

Codecov Report

Merging #47 (c52a437) into main (5bd3cec) will decrease coverage by 2.01%. The diff coverage is 56.38%.

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
- Coverage   83.80%   81.79%   -2.02%     
==========================================
  Files          11       11              
  Lines        1155     1247      +92     
==========================================
+ Hits          968     1020      +52     
- Misses        129      167      +38     
- Partials       58       60       +2     

Impacted Files	Coverage Δ
client.go	66.66% <56.38%> (-3.37%)	:arrow_down:

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