CampaignRequest: Change end datetime format and add getset

[Pelle-Jensen]

Since creating an agreement requires a CampaignRequest instance to be able to specify a campaign and i couldn't find any interface for CampaignRequest i added getters and setters to the class.
Also changed the date time formatting to match up with the rest of the package.

[codecov-commenter]

Codecov Report

Merging #40 (a44dba3) into 2.x (cb4b837) will decrease coverage by 0.75%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##                2.x      #40      +/-   ##
============================================
- Coverage     61.20%   60.45%   -0.76%     
- Complexity      436      440       +4     
============================================
  Files            79       80       +1     
  Lines          1441     1459      +18     
============================================
  Hits            882      882              
- Misses          559      577      +18     

Flag	Coverage Δ
unittests	60.45% <0.00%> (-0.76%)	⬇️

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Impacted Files	Coverage Δ
src/Model/RecurringPayment/CampaignRequest.php	0.00% <0.00%> (ø)

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[zaporylie]

Thanks!