Extracts PSR-17/18 discovery integration and unit tests functionality

[gsteel]

Extracts discovery methods into an internal trait and unit tests it.

[codecov]

Codecov Report

Merging #3 (643f463) into main (107632f) will increase coverage by 4.48%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main       #3      +/-   ##
============================================
+ Coverage     27.30%   31.78%   +4.48%     
  Complexity      215      215              
============================================
  Files            18       19       +1     
  Lines           791      799       +8     
============================================
+ Hits            216      254      +38     
+ Misses          575      545      -30     

Impacted Files	Coverage Δ
src/Postmark/PostmarkClientBase.php	0.00% <ø> (ø)
src/Postmark/ClientBehaviour/Discovery.php	100.00% <100.00%> (ø)
src/Postmark/Exception/DiscoveryFailure.php	100.00% <0.00%> (+100.00%)	⬆️

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