fix: fix rvalue for addmember/pushback

[liuq19]

AddMember/Pushback should use the rvalue argument because the value args will be moved.

[codecov-commenter]

Codecov Report

Merging #57 (3488eb9) into master (780c40c) will increase coverage by 0.07%. The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
+ Coverage   95.04%   95.11%   +0.07%     
==========================================
  Files          22       22              
  Lines        2785     2785              
==========================================
+ Hits         2647     2649       +2     
+ Misses        138      136       -2     

Impacted Files	Coverage Δ
include/sonic/dom/genericnode.h	99.63% <100.00%> (ø)
include/sonic/experiment/lazy_update.h	88.46% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[xiegx94]

Also PushBack API.

[xiegx94]

LGTM