ElementMap step support

[lidongze0629]

What do these changes do?

gremlin> g.V(1,2,4).elementMap()
==>{label=person, id=1, name=marko, id=1, age=29}
==>{label=person, id=2, name=vadas, id=2, age=27}
==>{label=person, id=4, name=josh, id=4, age=32}

gremlin> g.V(1,2,4).elementMap("name", "age")
==>{age=27, label=person, id=2, name=vadas}
==>{age=32, label=person, id=4, name=josh}
==>{age=29, label=person, id=1, name=marko}

gremlin> g.V().elementMap()
==>{label=person, id=2, name=vadas, id=2, age=27}
==>{label=software, id=72057594037927941, name=ripple, id=5, lang=java}
==>{label=person, id=1, name=marko, id=1, age=29}
==>{label=software, id=72057594037927939, name=lop, id=3, lang=java}
==>{label=person, id=4, name=josh, id=4, age=32}
==>{label=person, id=6, name=peter, id=6, age=35}

Compare to the official result, now g.E().elementMap() lack the information of in(out)'s vertices

# our result
gremlin> g.E().elementMap()
==>{label=knows, id=0, start_id=0.5, end_id=0.5, weight=0.5}

# official result
gremlin> g.E().elementMap()
==>{label=knows, id=0, start_id=0.5, end_id=0.5, weight=0.5, IN=[label=person, id=0], OUT=[label=person, id=1]}

Related issue number

Fixes #2778

[codecov-commenter]

Codecov Report

Merging #2942 (cf2cb06) into main (badf3f3) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2942   +/-   ##
=======================================
  Coverage   42.31%   42.31%           
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  Files          99       99           
  Lines       10631    10631           
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  Hits         4499     4499           
  Misses       6132     6132           

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