For plugins, only enable color output if command is running in a terminal

[vcheung-stripe]

Reviewers

r? @
cc @stripe/developer-products

Summary

This PR changes the plugin interface so that the host tells the plugin process if the host's stdin/out/err are terminals or not. Plugins can read this this to decide if they should print color output. In addition, this opens up the possibility of adding other terminal-only features, evolving the plugin interface, and separating concerns between host and plugin.

This change is needed because functions like term.IsTerminal always return false when called inside the plugin due to how the plugin's file descriptors are managed by the host process.

To change the interface, we leverage the go-plugin package's versioning ability to implement a "major change" to the plugin interface. The old interface is defined as:

type Dispatcher interface {
	RunCommand(args []string) (string, error)
}

while the new interface is defined as:

type DispatcherGRPC interface {
	RunCommand(additionalInfo *proto.AdditionalInfo, args []string) error
}

This new interface uses gRPC instead of net/rpc to handle IPC between the host and plugin. gRPC is needed because the net/rpc implementation can't handle serializing complex data structures, which we've introduced in a struct called AdditionalInfo.

This change is backwards compatible because the go-plugin library automatically handles version negotiation. Although the new interface is backwards incompatible, we keep the old interface around so older clients can continue to request the old interface.