Updated FlattenedStreamId to accept subclasses of Iterable

[LaMonF]

Currently FlattenedStreamId accepts a source stream id as "StreamId<Iterable<? extends T>>". That creates a problem if we want to pass "StreamId<List<? extends T>>". Therefore we changed Iterable to "? extends Iterable".

[codecov]

Codecov Report

Merging #10 into master will not change coverage.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master      #10   +/-   ##
=========================================
  Coverage     49.36%   49.36%           
  Complexity      248      248           
=========================================
  Files            72       72           
  Lines          1108     1108           
  Branches        158      158           
=========================================
  Hits            547      547           
  Misses          500      500           
  Partials         61       61

Impacted Files	Coverage Δ	Complexity Δ
.../service/streamfactory/FlattenedStreamFactory.java	100% <100%> (ø)	5 <1> (ø)	⬇️
...gpool/core/service/streamid/FlattenedStreamId.java	21.73% <100%> (ø)	3 <1> (ø)	⬇️

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