Add a public-facing API

[zorun]

We need an API for bird-lg. It would allow to better separate the web UI and the backend, and allow external tools to query bird-lg.

This is about a public API that would be exposed by bird-lg, and not about router <--> bird-lg communication (routers should not expose a publicly-available interface anyway)

The API should:

• use a standard format (JSON)
• cover everything that is currently possible in the web UI (i.e. we could rewrite the web frontend to use the API)
• if possible, adhere to some "standard" for this kind of API

[zorun]

Here is some initial research about existing looking glass APIs (from #35) :

There have been upstream discussions about structured output in Bird: https://bird.network.cz/pipermail/bird-users/2017-June/011356.html
And a patch from Netflix enabling JSON output directly in Bird: https://bird.network.cz/pipermail/bird-users/2018-March/012088.html

There's also Periscope from CAIDA, that parses many kind of looking glasses and outputs JSON:
https://www.caida.org/tools/utilities/looking-glass-api/
https://www.nanog.org/sites/default/files/20161018_Giotsas_Periscope_Standardizing_And_v1.pdf

[zorun]

The "Bird's Eye" project from INEX also has an API: https://github.com/inex/birdseye

Example here: https://www.inex.ie/rc1-cork-ipv4/