Move required dependencies to Composer require

[imshashank]

Fixes #1285
Fixes #1341

[codecov-io]

Codecov Report

Merging #1342 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1342      +/-   ##
============================================
- Coverage     91.87%   91.86%   -0.02%     
- Complexity     2466     2471       +5     
============================================
  Files           142      142              
  Lines          7523     7544      +21     
============================================
+ Hits           6912     6930      +18     
- Misses          611      614       +3

Impacted Files	Coverage Δ	Complexity Δ
src/Sdk.php	87.27% <0%> (-0.97%)	19% <0%> (+5%)

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[kstich]

This needs a Changelog Document. Because this is an update to the core of the SDK in terms of requirements, it should be messaged as a "feature" sized update to the core in the changelog document.

This was a misclassification going all the way back to v3.0.0 - it's even inconsistent with the associated compatibility-test file in terms of the required extensions. Since we don't have hard requirements on curl and dom, these should be moved to suggested installs in the compatibility test file as well.

[imshashank]

@kstich Updated the test and compatibility code, please let me know if anything else required.