Custom meta tags allows multiple entries for the same path

Problem/Motivation

Custom meta tags allows multiple entries for the same path.

Before adding a new path it should check, if there's already an entry for the exact same path (unique check)

Steps to reproduce

Go to /admin/config/search/metatag/custom/create and enter a custom path, for example /search
Configure and save the path
Go to /admin/config/search/metatag/custom/create and enter the same custom path again
See that now two entries with the same path exist in the overview

Proposed resolution

Ensure paths are unique

Remaining tasks

User interface changes

API changes

Data model changes