feat: implement specific API for duplicating form fields

[chowyiyin]

Problem

This PR creates a new endpoint POST /api/v3/admin/forms/:formId/fields/:fieldId/duplicate and extracts the functionality to duplicate form fields from the existing endpoint PUT /:formId/adminform.

Closes #1483

Solution

• admin-forms.form.routes.ts: Added new endpoint
• admin-form.controller.ts: Added handleDuplicateFormField controller
• admin-form.service.ts: Added duplicateFormField service
• form-server-model.ts: Added duplicateFormFieldById
• • Duplication of the existing field is done here
• UPDATE_FORM_TYPES: Added DuplicateField
• admin-form.client.controller.js: Added another case for duplication to call new API and update view

Tests

Unit Tests

• admin-form.controller.specs.ts: add unit tests handleDuplicateFormField
• admin-form.service.specs.ts: add unit tests for duplicateFormField
• admin-forms.form.routes.spec.ts: add unit tests forPOST /:formId/fields/:fieldId/duplicate
• form.server.model.spec.ts: add unit tests for duplicateFormFieldById instance method

Manual Tests

• Create random field in storage mode and duplicate it. Check that contents of field are duplicated correctly.
• Create random field in email mode and duplicate it. Check that contents of field are duplicated correctly.
• Create myinfo field (with dropdown options) and duplicate it. Check that duplicated field has the correct options and is also uneditable if it has a pre-filled value.

[seaerchin]

looks good! but add docs cos some stuff abit confusing

[mantariksh]

lgtm, to test on staging before approval. also requesting @karrui and @tshuli to take another look if you have time!

[karrui]

lgtm after fixing typos