Add convenience function to return the size of an encrypted segment

[pontus]

Add straight-forward function, given a header/private key that can decrypt it, return the encrypted segment size used in the stream.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 69.23077% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 71.21%. Comparing base (adf6e1f) to head (27d98b6).

Files	Patch %	Lines
model/headers/headers.go	69.23%	2 Missing and 2 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
+ Coverage   69.75%   71.21%   +1.45%     
==========================================
  Files           6        6              
  Lines         873      886      +13     
==========================================
+ Hits          609      631      +22     
+ Misses        141      135       -6     
+ Partials      123      120       -3     

Flag	Coverage Δ
unittests	71.21% <69.23%> (+1.45%)	⬆️

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