support ngql reader

[Nicole00]

What type of PR is this?

• bug
• feature
• enhancement

What problem(s) does this PR solve?

Issue(s) number:

Description:

support read edge data through the query statement.

How do you solve it?

use just one spark partition to execute the query, get the edge data in ResultSet by iterator. And convert the edge Value in query ResultSet to Spark DataFrame edge.

Special notes for your reviewer, ex. impact of this fix, design document, etc:

the query is not distributed in spark layer. Please make sure the query's result is edges result. For the usage, please see NebulaSparkReaderExample.scala

[codecov-commenter]

Codecov Report

Base: 50.54% // Head: 44.67% // Decreases project coverage by -5.87% ⚠️

Coverage data is based on head (843229f) compared to base (0e037b7).
Patch coverage: 7.52% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #78      +/-   ##
============================================
- Coverage     50.54%   44.67%   -5.88%     
  Complexity      123      123              
============================================
  Files            27       30       +3     
  Lines          1460     1672     +212     
  Branches        241      280      +39     
============================================
+ Hits            738      747       +9     
- Misses          645      846     +201     
- Partials         77       79       +2     

Impacted Files	Coverage Δ
...la/com/vesoft/nebula/connector/NebulaOptions.scala	0.00% <0.00%> (ø)
...nnector/reader/NebulaNgqlEdgePartitionReader.scala	0.00% <0.00%> (ø)
...soft/nebula/connector/reader/NebulaPartition.scala	66.66% <0.00%> (-33.34%)	⬇️
...t/nebula/connector/reader/NebulaSourceReader.scala	85.71% <0.00%> (-7.39%)	⬇️
...nebula/connector/reader/NebulaNgqlEdgeReader.scala	0.00% <0.00%> (ø)
...vesoft/nebula/connector/reader/NebulaNgqlRDD.scala	0.00% <0.00%> (ø)
...esoft/nebula/connector/reader/NebulaRelation.scala	87.50% <33.33%> (-3.81%)	⬇️
...ala/com/vesoft/nebula/connector/NebulaConfig.scala	56.86% <55.17%> (-0.03%)	⬇️

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