Improve plugin system

[mostafa]

• Add command arguments to plugin on load #72
• Pass env-vars to the plugin command on load #76
• Intercept traffic #89
• Hook types should be introduced. Currently there are two types: notification and normal hooks. Notification hooks are just to notify the plugin that an event happened, while the normal hooks have different purposes. This should be improved.
• Non-unary RPC calls should be investigated.
• The plugin config object should be read and somehow used. Currently it is just loaded into the PluginImpl.Config and not used. Other config parameters should be investigated as well.
• Write a proposal for the built-in plugins.
• Consider implementing all features from go-plugin as seems fit.
• Investigate usage of sigstore/cosign or the Grafana way for plugins
• Load plugin from a remote location, preferably GitHub
• Use enum/constant for hook names in protobuf

[mostafa]

A good chunk of tasks has been dealt with in milestone v0.2.x. The rest will be implemented or fixed on-demand.

[mostafa]

Moved all items to #46, so I'll close this ticket in favor of the other one.