Refactor AgentHeaderDescriptor

[michaelbausor]

• Replace AgentHeaderDescriptor with AgentHeader, a class with only static methods
• In GapicClientTrait, cache only the gapic version loaded from file
• Remove AgentHeaderMiddleware and use FixedHeaderMiddleware in its place

[tmatsuo]

Looks like there's a cs error, feel free to merge once it's fixed

[codecov-io]

Codecov Report

Merging #206 into master will increase coverage by 0.66%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
+ Coverage   84.14%   84.81%   +0.66%     
==========================================
  Files          63       62       -1     
  Lines        2826     2838      +12     
==========================================
+ Hits         2378     2407      +29     
+ Misses        448      431      -17

Impacted Files	Coverage Δ
src/ApiCore/AgentHeader.php	100% <100%> (ø)
src/ApiCore/Middleware/FixedHeaderMiddleware.php	100% <100%> (+100%)	⬆️
src/ApiCore/GapicClientTrait.php	88.88% <100%> (-0.44%)	⬇️
src/ApiCore/Version.php	66.66% <75%> (+23.8%)	⬆️
src/ApiCore/Serializer.php	91.89% <0%> (-1.05%)	⬇️
src/ApiCore/ApiException.php	90% <0%> (+15%)	⬆️

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