Introduce GetEmbeds() and SetEmbeds()

[wneessen]

These two methods work analogous to the *Attachments() methods and provide the user with access to the embedded files.

[codecov-commenter]

Codecov Report

Merging #109 (25e00c6) into main (c602ba1) will decrease coverage by 0.13%. The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main     #109      +/-   ##
==========================================
- Coverage   80.74%   80.62%   -0.13%     
==========================================
  Files          20       20              
  Lines        1844     1848       +4     
==========================================
+ Hits         1489     1490       +1     
- Misses        257      259       +2     
- Partials       98       99       +1     

Impacted Files	Coverage Δ
msg.go	86.74% <100.00%> (+0.08%)	:arrow_up:
client.go	79.26% <0.00%> (-1.01%)	:arrow_down:

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