Add Mode aggregation function

[Aritra8438]

Fixes #2918

This pull request adds support for Mode aggregation as a transformation step in the data explorer.

User will be able to:

• Get the mode of any column based on Group By clause.

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Technical details

I have made the following changes:

• Add Mode(DBFunction) class to support mode aggregation
• Add mode suffix to the output alias of the aggregated column.
• Add Mode to the list of transformation steps in the data explorer.
• Add test for the same.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the develop 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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[Aritra8438]

Hi, @mathemancer, @seancolsen, this PR is stacked on PR #2932.

[mathemancer]

Looks great; nice work!