Support custom operator in addCondition method

[alirezanet]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context.

Fixes #225

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• I have made corresponding changes to the documentation
• I have commented my code, particularly in hard-to-understand areas
• New and existing unit tests pass locally with my changes

[what-the-diff]

PR Summary

• Enhanced the addCondition method
  The functionality of the addCondition method is expanded to allow the input of operator parameter not only as ConditionalOperator but now even as a simple string. This provides versatility for users in the code input process.

• Implemented Validation for Custom Operators
  To ensure consistent code inputs, a validation feature has been added. This checks that any custom operators used begin with a # character. If the operators used do not conform to this rule, an error gets thrown. This allows for structure and conformity in operator use which enhances the readability and debugging process.

• Added More Rigorous Testing for Custom Operators
  Many new tests have been introduced into GridifyQueryBuilder.test.ts. These tests cover scenarios such as validating that operators beginning with # can be added without issues. Additionally, they also ensure that an error is indeed cast when operators without the # character are added. This testing is vital for supporting the new custom operator feature additions and ensuring code reliability.