Examples: datasource with support for annotations

[mckn]

Description

Create a plugin that contains a datasource with backend that returns static data regardless of the query parameters. This should be an example of how to create datasource that supports annotations.

It can use the: https://github.com/grafana/plugins-examples/issues/1 as a base but enhance it with support for annotations.

Tasks

• A annotations query editor.
  • Showcase how to use the grafana-ui form components.
• A datasource with backend
  • Showcase how to support annotations.
• Integration tests that verifies that the features that being used from Grafana plugin API is working as expected.