enumerate missing extensions

[ClaytonJY]

Rather than add one extension at a time as we find a need, we should try to do an exhaustive sweep through what Timescale Cloud makes available and try to support all the same extensions.

These can be split into two groups: those already available in the timescale/timescaledb base image, and those which need to be installed.

This will generate most issues: white-listing the already-installed extensions should be one easy issue, but those that need to be installed will require more work and could each be their own issue.

[ClaytonJY]

Listed out at https://github.com/everactive/timescale-cloud-docker/wiki/Timescale-Cloud-extensions---PG12; still needs to be categorized.