[DOP-6700] Add BaseFileDFConnection & BaseFileFormat classes

[dolfinus]

Change Summary

Added new base classes:

• BaseFileDFConnection
• BaseFileFormat

Implementation may change in future pull requests.

Also renamed existing methods of BaseDBConnection: read_df -> read_source_as_df, write_df -> write_df_as_files. This is breaking change, so upgraded version to 0.9.0

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

Merging #66 (230dcdd) into develop (d4c39da) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop      #66      +/-   ##
===========================================
+ Coverage    93.14%   93.16%   +0.01%     
===========================================
  Files          127      129       +2     
  Lines         6131     6146      +15     
  Branches      1149     1149              
===========================================
+ Hits          5711     5726      +15     
  Misses         322      322              
  Partials        98       98              

Impacted Files	Coverage Δ
onetl/base/base_db_connection.py	100.00% <ø> (ø)
onetl/base/__init__.py	100.00% <100.00%> (ø)
onetl/base/base_file_df_connection.py	100.00% <100.00%> (ø)
onetl/base/base_file_format.py	100.00% <100.00%> (ø)
onetl/connection/db_connection/greenplum.py	69.62% <100.00%> (ø)
onetl/connection/db_connection/hive.py	91.59% <100.00%> (ø)
onetl/connection/db_connection/jdbc_connection.py	95.81% <100.00%> (ø)
onetl/connection/db_connection/kafka/connection.py	91.20% <100.00%> (ø)
onetl/connection/db_connection/mongodb.py	94.09% <100.00%> (ø)
onetl/db/db_reader/db_reader.py	90.05% <100.00%> (ø)
... and 1 more