feat: domain apis

[mcarey1590]

Adds domain APIs from spec https://postmarkapp.com/developer/api/domains-api

[mergify]

Welcome to our open-source project! 💘

[codecov]

Codecov Report

Attention: Patch coverage is 50.66667% with 37 lines in your changes missing coverage. Please review.

Project coverage is 87.08%. Comparing base (1806d09) to head (90998cb).
Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
domains.go	50.66%	37 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
- Coverage   91.31%   87.08%   -4.24%     
==========================================
  Files          13       14       +1     
  Lines         645      720      +75     
==========================================
+ Hits          589      627      +38     
- Misses         48       85      +37     
  Partials        8        8              

Flag	Coverage Δ
unittests	87.08% <50.66%> (-4.24%)	⬇️

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Files with missing lines	Coverage Δ
domains.go	50.66% <50.66%> (ø)

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