Enforce `ALLOWED_URL_SCHEMES` for URLs in custom fields

### NetBox version v4.6.4 ### Feature type Change to existing functionality ### Proposed functionality Restrict custom field URLs to the schemes listed in [`ALLOWED_URL_SCHEMES`](https://netboxlabs.com/docs/netbox/configuration/security/#allowed_url_schemes). The whitelist of permitted schemes would be applied in addition to any validation regex associated with the field. ### Use case It may be desirable for `ALLOWED_URL_SCHEMES` to be enforced for custom fields as well. However, this is a potentially breaking change for many users, and should be considered carefully. ### Database changes N/A ### External dependencies N/A