Implementation for column mappings

[Anish9901]

Fixes #1508

This PR introduces the implementation of column mappings which will be used as a part of the import process while the records are to be imported from one table to another so that a user can exactly specify which column from the import file/temp_table should map to which specific columns of the target table.

Technical details

POST /api/db/v0/tables/<temp_table_id>/existing_import/

Request

{
    "import_target": <target_table_id>,
    "data_files": [<data_file_id>],
    "mappings": [
                    [<temp_table_col1_id>,<target_table_col1_id>],
                    [<temp_table_col2_id>,<target_table_col2_id>]
                ]
}

Checklist

• [x] My pull request has a descriptive title (not a vague title like Update index.md).
• [x] My pull request targets the master branch of the repository
• [x] My commit messages follow best practices.
• [x] 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).
• [x] I tried running the project locally and verified that there are no visible errors.

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[codecov-commenter]

Codecov Report

Merging #1506 (1433840) into master (01a4bdd) will increase coverage by 0.02%. The diff coverage is 53.33%.

@@            Coverage Diff             @@
##           master    #1506      +/-   ##
==========================================
+ Coverage   92.49%   92.52%   +0.02%     
==========================================
  Files         142      142              
  Lines        6305     6314       +9     
==========================================
+ Hits         5832     5842      +10     
+ Misses        473      472       -1     

Flag	Coverage Δ
pytest-backend	92.52% <53.33%> (+0.02%)	:arrow_up:

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

Impacted Files	Coverage Δ
mathesar/models/base.py	92.55% <0.00%> (-0.84%)	:arrow_down:
mathesar/api/serializers/tables.py	89.04% <40.00%> (-1.87%)	:arrow_down:
db/records/operations/insert.py	95.74% <100.00%> (+16.19%)	:arrow_up:

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