tel: URL scheme for the URL field (and others?)

I would like to store phone number in KeePassDX, so that I can click-call numbers without the need to store them into contacts.
With a `kbdx` file being synchronized between devices in many usecases, and even from a privacy perspective, to maintain phone numbers with entries would be convenient for me as a user. And passphrase-secured encryption on top! (y)

When I enter into the existing "hyperlink-aware" field `URL` the value

> tel:+1234567890

then the field turns red with the error message "Invalid URL".
As a URL scheme it is valid though (RFC 3966).
**IT IS possible to store** the entry though.

I also experimented with Advanced Fields, and they have the same behavior.

In the Android app I can long-press the field content, and it cleverly selects the number without the tel: prefix.
Even suggesting then to call. So the minimal requirements to sync phonenumbers is already working.

**The question might arise if a password manager should deal with phonenumbers.** Well, the first time I tried was when I wanted to add to my enty with the 'health insurance webpage login' the local service line phone number. And I feel that any multi-channel support would give me a customer login as well as a callcenter line -- despite the trend to self service. I clearly have many entries were a phonenumber is related.

And honestly: I am looking for a well-rounded solution to keep my Android contacts as empty as possible. Screw those home-phoning apps!

The issue does not name specific files or tests. Start by locating the URL validation used by the URL field and Advanced Fields, then verify that the `tel:` scheme is accepted without the “Invalid URL” error in both places.

Written by the indexing model from the issue text.