feat: add eq, ne, gt, gte, lt, lte subquery methods

[AkshitShandilya]

PR Info

• Closes Other operators for sub-queries (gte, eq, in, ...) #951
• Dependencies:
  • N/A
• Dependents:
  • N/A

New Features

• Added eq_subquery, ne_subquery, gt_subquery, gte_subquery, lt_subquery, lte_subquery methods to ExprTrait for comparing expressions against subqueries using =, <>, >, >=, <, <= operators respectively

Bug Fixes

• N/A

Breaking Changes

• N/A

Changes

• Each new method follows the same pattern as the existing in_subquery and not_in_subquery methods
• Doc examples included for MySQL, Postgres, and SQLite backends