Use hashicorp/go-plugin to allow plugins to connect via RPC/gRPC

[uberswe]

Use https://github.com/hashicorp/go-plugin

After playing around with different plugin options I think the plugin package from hashicorp will offer the most flexibility for Beubo without many downsides. The biggest downside is performance so when implementing this, some consideration will need to be put towards the number of calls per plugin. We could consider still supporting the standard go plugin package if they can be kept general enough.

Benchmarks: https://github.com/uberswe/go-plugin-benchmark