cli: refactor `show ospf [cmd]` from vtysh commands to proper operational data

[troglobit]

As discussed in #1155, specifically here, we should continue the refactor of CLI commands into proper operational data.

Quoting @rical:

The general plan here is to avoid execution of arbitrary shell commands and rely solely on operational data though the new show command. One reason for this is to protect against someone breaking out of the CLI and NACAM though command insertion (think > show interface eth0;/bin/bash)

So ideally we should avoid printing info in the CLI directly from vtysh, instead we should use show which uses operational data, which means we need to populate this data in the operational data.

---

The tricky bit is that there's a lot of operational data in OSPF that one might be interested, and to make matters worse, we've seen in the past that state/data available in the pretty-printed vtysh show commands has not always been available (because of Frr bugs) in the JSON output for the same command.

Nevertheless, customers querying status remotely, as well as ourselves for the WebUI, expect all the data to be available for debugging/metrics/supervision.

[troglobit]

After discussions in the team, we've decided to temporarily drop CLI support for show ospf database, because it's a LOT of data that needs to be extracted and not all is supported in the existing YANG model(s). For advanced debugging we believe the vtysh "escape hatch" is sufficient for now. See #1253