Add an option to store the json directly in the datasource configuration

[mlcdf]

Hello Marcus,

We're considering using this plugin at work, but since any HTTP calls can be performed, we're worried it'll be misused:

• people are going to make HTTP calls to fetch dynamic stuff they can get via regular metrics
• it'll create a lot of unspoken dependencies between apps/servers and Grafana (ie X dashboards will required access to Y and Z HTTP endpoints).

One simple solution to this issue would be to store the json directly inside Grafana, in the datasource configuration, via a new textarea field "Raw JSON". And we think this might interest other orgs or people as well. :)

What do you think? Is this in the scope of this plugin? If yes, would you accept a PR?

Maxime