expose labelsOffset/verticalMargins for EKAlertMessageView.

[tospery]

Like:
let alertMessageView = EKAlertMessageView.init(with: alertMessage)
alertMessageView.messageContentView.verticalMargins = 30
alertMessageView.messageContentView.labelsOffset = 15

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 2.04%. Comparing base (cf1cde7) to head (db5e970).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
Source/MessageViews/EKSimpleMessageView.swift	0.00%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #354   +/-   ##
======================================
  Coverage    2.04%   2.04%           
======================================
  Files          96      96           
  Lines        5472    5472           
======================================
  Hits          112     112           
  Misses       5360    5360           

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