[feature] support key by url in apt configure module

This bug was originally filed in Launchpad as [LP: #2006775](https://bugs.launchpad.net/cloud-init/+bug/2006775)

Launchpad details

affected_projects = []

assignee = None

assignee_name = None

date_closed = None

date_created = 2023-02-09T19:08:55.982578+00:00

date_fix_committed = None

date_fix_released = None

id = 2006775

importance = wishlist

is_complete = False

lp_url = https://bugs.launchpad.net/cloud-init/+bug/2006775

milestone = None

owner = holmanb

owner_name = Brett Holman

private = False

status = triaged

submitter = holmanb

submitter_name = Brett Holman

tags = ['bitesize']

duplicates = []

_Launchpad user **Brett Holman(holmanb)** wrote on 2023-02-09T19:08:55.982578+00:00_

A common way of distributing keys is by hosting them at a URL for download. This is not currently supported by the apt configure module, and would be a simple addition.

Example usage (note the suggested 'keyurl' key)

```yaml
apt:
sources:
source1:
keyurl: 'https://domain.io/keys/key1.gpg'
source: 'deb [signed-by=$KEY_FILE] http:/// jammy main'
```