Swift Package Manager integration improvements

[danielrbrowne]

This PR resolves #161

[codecov-io]

Codecov Report

Merging #162 into master will decrease coverage by 0.36%.
The diff coverage is 89.13%.

@@            Coverage Diff             @@
##           master     #162      +/-   ##
==========================================
- Coverage   90.09%   89.72%   -0.37%     
==========================================
  Files          57       57              
  Lines        2898     2921      +23     
==========================================
+ Hits         2611     2621      +10     
- Misses        287      300      +13     

Impacted Files	Coverage Δ
Tests/Internal/ServerSyncProcessHandlerTests.swift	97.56% <88.09%> (+<0.01%)	⬆️
Tests/BeamsTokenProviderTests.swift	84.21% <100.00%> (ø)
Sources/ServerSyncJob.swift	60.82% <0.00%> (-8.25%)	⬇️
Sources/Internal/ServerSyncProcessHandler.swift	80.23% <0.00%> (-1.98%)	⬇️
Tests/RegisterTests.swift	100.00% <0.00%> (ø)
Tests/InterestsTests.swift	100.00% <0.00%> (ø)
Sources/ServerSyncEventHandler.swift	100.00% <0.00%> (ø)
Tests/IntegrationTests/StopTests.swift	100.00% <0.00%> (ø)
Tests/IntegrationTests/SetUserIdTest.swift	100.00% <0.00%> (ø)
Tests/IntegrationTests/ClearAllStateTest.swift	100.00% <0.00%> (ø)
... and 9 more

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[callum-oakley]

usual disclaimer about me not really knowing swift, but this all looks reasonable to me