[feature] Add `target_url_suffix` for notification target URLs

[nemesifier]

Is your feature request related to a problem? Please describe.

We need a way to append a querystring or fragment to a notification target URL, especially for generic_message notifications, without allowing callers to override the full URL.

Describe the solution you'd like

Add a target_url_suffix argument to notify.send, for example:

notify.send(
    type="generic_message",
    sender=obj,
    target=obj,
    target_url_suffix="?status=pending",
)

The suffix should be appended in the notification model when resolving target_url. It should also support fragments, for example target_url_suffix="#pending".

Invalid values should raise ValueError when passed to notify.send. Valid suffixes should start with ?, &, or #.

Describe alternatives you've considered

Using the existing notification type target_link mechanism works for dedicated notification types, but it is type-level and not suitable for per-notification generic_message usage.

Allowing a full URL override is too broad for this use case.

Additional context

The goal is to keep the feature narrow and model-centered: generate the existing target URL as usual, then append the validated suffix.

Originally needed in openwisp/openwisp-firmware-upgrader#436.