fix: prevent adding multiple empty rows

[DenserMeerkat]

PR Description

Resolved the issue of multiple rows being added by introducing an isAddingRow flag. The flag prevents additional rows from being added while the user is still typing rapidly.

https://github.com/foss42/apidash/assets/95911940/b832fbf0-46e4-43cd-9651-c53585b696af

Related Issues

• Related Issue #345
• Closes #345

Checklist

• [X] I have gone through the contributing guide
• [X] I have updated my branch and synced it with project main branch before making this PR
• [X] I have run the tests (flutter test) and all tests are passing

Added/updated tests?

We encourage you to add relevant test cases.

• [ ] Yes
• [X] No, and this is why: Doesn't require/break any tests.

[animator]

Thanks for the fix. But, there is one issue .. which might require raising a new issue & PR. After adding a lot of rows, whenever a new row gets added next, the current textfield loses focus & the table scrolls to the top making it inconvenient for the user.

[animator]

Raised issue #350