AtlasSemantics: Fix VarHandleArraySemantics for default value append

Merged Frank Winklmeier requested to merge fwinkl/athena:vhka_semantics into 23.0

The following code resulted in a modified default value for all properties following VarHandleArraySemantics:

   MyAlg.MyVKHArray += ['foo']

Fix this by using SequenceSemantics and add unit tests.

Closes ATEAM-902 and relates to ATR-27311.

Edited by Frank Winklmeier

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