chore: modify and add test for metaframe loader; add inits for codecov.

[rsyi]

Addresses the following issues:

#8
#9
#10

[codecov-commenter]

Codecov Report

Merging #18 into master will increase coverage by 49.72%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master      #18       +/-   ##
===========================================
+ Coverage    0.00%   49.72%   +49.72%     
===========================================
  Files          15       15               
  Lines         549      549               
===========================================
+ Hits            0      273      +273     
+ Misses        549      276      -273     

Impacted Files	Coverage Δ
metaframe/loader/metaframe_loader.py	92.85% <ø> (+92.85%)	⬆️
metaframe/utils/neo4j.py	11.11% <0.00%> (+11.11%)	⬆️
metaframe/models/connection_config.py	16.66% <0.00%> (+16.66%)	⬆️
...ame/extractor/amundsen_neo4j_metadata_extractor.py	18.18% <0.00%> (+18.18%)	⬆️
metaframe/engine/presto_engine.py	32.47% <0.00%> (+32.47%)	⬆️
metaframe/utils/extractor_wrappers.py	34.61% <0.00%> (+34.61%)	⬆️
metaframe/models/presto_watermark.py	47.05% <0.00%> (+47.05%)	⬆️
metaframe/utils/connections.py	50.00% <0.00%> (+50.00%)	⬆️
metaframe/engine/sql_alchemy_engine.py	62.85% <0.00%> (+62.85%)	⬆️
metaframe/engine/base_engine.py	75.00% <0.00%> (+75.00%)	⬆️
... and 5 more

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