[DOP-8528] Allow modes "ignore" and "error" in GreenplumWriteOptions

[maxim-lixakov]

Change Summary

• add ignore and error modes (if_exists=...) to Greenplum.WriteOptions
• add unit and integration tests
• updated documentation

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 #142 (1a7f2b7) into develop (ebbeed2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #142   +/-   ##
========================================
  Coverage    94.29%   94.29%           
========================================
  Files          207      207           
  Lines         7697     7699    +2     
  Branches      1451     1451           
========================================
+ Hits          7258     7260    +2     
  Misses         320      320           
  Partials       119      119           

Files Changed	Coverage Δ
...l/connection/db_connection/greenplum/connection.py	70.38% <100.00%> (ø)
...netl/connection/db_connection/greenplum/options.py	100.00% <100.00%> (ø)

[dolfinus]

Добавь запись в docs/changelog/next_release, по аналогии с существующими там файлами