Prepare for mailgun 2.3

[Nyholm]

We want to support the new mailgun API. There are still some issue here. But this is a first draft.

[Nyholm]

I've solved the issues. This PR is ready for a review. It is blocked by mailgun/mailgun-php#341

[codecov-io]

Codecov Report

Merging #40 into master will decrease coverage by 0.04%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
- Coverage   88.11%   88.07%   -0.05%     
==========================================
  Files           3        3              
  Lines         101      109       +8     
==========================================
+ Hits           89       96       +7     
- Misses         12       13       +1

Impacted Files	Coverage Δ
Service/MailgunTransport.php	84.5% <66.66%> (+1.17%)	⬆️
...encyInjection/cspooSwiftmailerMailgunExtension.php	92.3% <83.33%> (-7.7%)	⬇️

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