Include TimescaleDB Toolkit

Many, if not most, useful [timescaledb hyperfunctions require timescaledb-toolkit](https://docs.timescale.com/keywords/Toolkit/) which is included in their managed offering.
It would be great for the timescaledb image to include it. Currently this requires building a custom image which is not ideal.

Fortunately it is quite [easy to add via a Debian package](https://docs.timescale.com/self-hosted/latest/tooling/install-toolkit/#install-toolkit-on-ubuntu-and-other-debian-based-systems) and [available on the packagecloud timescale repository](https://packagecloud.io/app/timescale/timescaledb/search?q=timescaledb-toolkit)

The [Enabling TimescaleDB docs](https://fly.io/docs/postgres/getting-started/enabling-timescale/) should also note [enabling/updating the extension](https://docs.timescale.com/self-hosted/latest/tooling/install-toolkit/#install-and-update-toolkit-on-managed-service-for-timescaledb)