Replace column index usage in db create operations with column attnum.

[silentninja]

Fixes #995

Breaking Change

• Column Duplication uses column id instead of column index. So source_column field in the request body of POST method of /column/ accepts column id instead of column index

Other Changes

• Column index usage has been completely removed from column create operations and from certain column alter operations as they were being used by create operations.

• get_columns_name_from_attnums returns a list of column attnums instead of a tuple of a row tuple with single attnum attribute

• get_column_name_from_attnum convenience function has been introduced to help with fetching attnum for single column from column name

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the master branch of the repository
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[codecov-commenter]

Codecov Report

Merging #1110 (351797e) into mathesar-993-attnum-column-drop (f7029fb) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@                         Coverage Diff                         @@
##           mathesar-993-attnum-column-drop    #1110      +/-   ##
===================================================================
- Coverage                            92.84%   92.79%   -0.05%     
===================================================================
  Files                                  112      112              
  Lines                                 4037     4052      +15     
===================================================================
+ Hits                                  3748     3760      +12     
- Misses                                 289      292       +3     

Flag	Coverage Δ
pytest-backend	92.79% <100.00%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
db/columns/operations/alter.py	96.57% <100.00%> (+0.12%)	⬆️
db/columns/operations/create.py	90.29% <100.00%> (+0.29%)	⬆️
db/columns/operations/select.py	100.00% <100.00%> (ø)
db/constraints/operations/create.py	95.45% <100.00%> (+1.01%)	⬆️
db/constraints/operations/select.py	97.72% <100.00%> (ø)
mathesar/api/db/viewsets/columns.py	91.01% <100.00%> (-0.39%)	⬇️
mathesar/api/serializers/columns.py	98.79% <100.00%> (ø)
mathesar/models.py	96.12% <100.00%> (ø)
...i/exceptions/generic_exceptions/base_exceptions.py	80.00% <0.00%> (-10.00%)	⬇️

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[silentninja]

@seancolsen SInce you are working on #1047, will you be able to fix these changes too? I can extract the breaking changes to a different PR if it will take time to fix on the frontend so as to not block other parts of the PR.

[seancolsen]

@silentninja the front end doesn't yet have any functionality to duplicate columns and it doesn't make use of source_column. So we should be all set.

[kgodey]

Looks good to me, please fix merge conflicts and merge.