Add Annotation interface for ExecSegment

[michaelschlies]

fixes #1192

Please see note in #1192 before merging, this hasn't been added to any places NonBlocking is presently.

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This change is 

[codecov-io]

Codecov Report

Merging #1474 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1474   +/-   ##
=========================================
  Coverage     73.76%   73.76%           
- Complexity     4551     4553    +2     
=========================================
  Files           652      652           
  Lines         17017    17017           
  Branches       1428     1428           
=========================================
  Hits          12552    12552           
  Misses         3843     3843           
  Partials        622      622

Impacted Files	Coverage Δ	Complexity Δ
...ile/internal/CachingFileSystemChecksumService.java	72.72% <0%> (-9.1%)	2% <0%> (-1%)
.../src/main/java/ratpack/reactor/ReactorRatpack.java	81.48% <0%> (-5.56%)	17% <0%> (ø)
...in/java/ratpack/exec/internal/CachingUpstream.java	88.57% <0%> (+1.42%)	20% <0%> (+1%)	⬆️
...ack/exec/util/internal/DefaultReadWriteAccess.java	84.55% <0%> (+1.62%)	29% <0%> (+1%)	⬆️
...a/ratpack/stream/internal/ManagedSubscription.java	70.83% <0%> (+2.08%)	21% <0%> (+1%)	⬆️

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