reading plugin settings from environment variable

[yesoreyeram]

This PR adds couple of utility methods to pluginContext which allow you to read plugin settings from environment variable.

This will also ensure the environment variable keys are consistent across the plugins in future. ( example: oracle uses this and azure data explorer uses this )

[andresmgot]

We had an offline conversation about this but as a summary:

• Using environment variables is the legacy approach so we should defer to use Grafana config instead.
• The format for env vars should be GF_PLUGIN_<config>.
• The place to extend the current config passed down to plugins is here in Grafana and here in the plugin SDK for new helper functions.