feat: Formula-based Final Score calculation in Appraisals

[vinyselopal]

Description: Added field in Appraisal Cycle to calculate final score for an employee appraisal using custom formula. Checking the field makes "Final Score Formula" field visible wherein you can add a Python Expression using autocompletion suggestions for various Employee master, Appraisal and Appraisal Cycle fields. This will automatically calculate the final score when creating new Appraisal

Closes #690

docs: https://frappehr.com/docs/v14/en/appraisal-cycle