Expose option for TCP keep alive

[ldaley]

This change is 

[codecov-commenter]

Codecov Report

Patch coverage: 37.14% and project coverage change: +0.04% 🎉

Comparison is base (093091b) 73.27% compared to head (c8323b9) 73.31%.
Report is 12 commits behind head on 1.10.x.

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Additional details and impacted files

@@             Coverage Diff              @@
##             1.10.x    #1679      +/-   ##
============================================
+ Coverage     73.27%   73.31%   +0.04%     
- Complexity     4830     4837       +7     
============================================
  Files           695      695              
  Lines         18366    18372       +6     
  Branches       1518     1521       +3     
============================================
+ Hits          13457    13470      +13     
+ Misses         4211     4203       -8     
- Partials        698      699       +1     

Files Changed	Coverage Δ
...ore/src/main/java/ratpack/server/ServerConfig.java	100.00% <ø> (ø)
.../main/java/ratpack/server/ServerConfigBuilder.java	40.00% <ø> (ø)
...xec/src/main/java/ratpack/exec/ExecController.java	100.00% <ø> (+25.00%)	⬆️
...ctor/internal/BlockingExecutorBackedScheduler.java	0.00% <0.00%> (ø)
.../rx2/internal/BlockingExecutorBackedScheduler.java	0.00% <0.00%> (ø)
...ient/internal/ContentAggregatingRequestAction.java	65.00% <9.09%> (-21.21%)	⬇️
...client/internal/ContentStreamingRequestAction.java	80.80% <10.00%> (-9.08%)	⬇️
...ck/exec/internal/DefaultExecControllerBuilder.java	52.94% <50.00%> (-11.77%)	⬇️
.../internal/module/ServerConfigDataDeserializer.java	91.54% <100.00%> (+5.24%)	⬆️
.../ratpack/server/internal/DefaultRatpackServer.java	86.77% <100.00%> (+0.67%)	⬆️
... and 5 more

... and 5 files with indirect coverage changes

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