Setup Go modules

[kylebrandt]

Once we have the gel/folder sane again and not importing a bunch of stuff we won't need, we should run go mod init and vendor.

We should also decide if we want the vendor folder in the repo. Personally that is my preference but I don't have a strong opinion (or much experience with go modules) . Also maybe that is a problem with how we normally do plugins?

[kylebrandt]

I think I basically have it working in the dsPlug branch with #15

Needed some require statements to get gRPC version match - and to target the bidirectional_poc branches of grafana and the plugin-repo