feat: introduce vervet-underground

[Todai88]

Readme should tell us what we want.

[CLAassistant]

All committers have signed the CLA.

[jcsackett]

LGTM!

Using config.json for configuration is a little odd in a go app, isn't it? Is that a company standard rather than the more common yaml or toml config paths most go projects seem to pursue?

[Todai88]

LGTM!

Using config.json for configuration is a little odd in a go app, isn't it? Is that a company standard rather than the more common yaml or toml config paths most go projects seem to pursue?

I'm not sure if it's a company standard, but the config library that we created in V3 suggests it maybe is.
Arguably JSON is a bit easier to parse than YAML, but I'm sure we could extend the lib to use YAML too, if we wanted to :)