Add eye-catcher to telemetry packet

### Description

With the release of the DigitalOcean droplet, we need a way to track usage of it in our telemetry.

We have often talked about adding an eye-catcher to the telemetry packet - a string that can be set via `flowforge.yml` and gets passed straight through. With that we could add a suitable value for the DO droplet image (and likewise for other marketplace installs we create in the future). One downside here is the value would be editable by the user - so could be modified post-install.

An alternative would be to identify some other property that distinguishes the platforms with some future-proofing for where else we find ourselves running.

Assuming the eye-catcher is the way forward, the tasks would be:

- [ ] Update telemetry code to check for the existence of `telemetry.label` in `flowforge.yml` and include in ping payload as `label`.
- [ ] Update ping collector to accept the value and write to db

We should limit it to 64 chars (32 even?) as this is user-injectable text.

### Epic/Story

_No response_