Need to be able to attach a debugger to subprocesses.

[joshuarowe-salesforce]

See #20466 in terraform.

I'm working on some changes to rpc_client.go and client.go to implement this.

[joshuarowe-salesforce]

I'm proposing adding a *NetRpcConfig member to the ClientConfig struct that looks like:

// NetRpcConfig allows providing properties of an underlying connection
// via the ClientConfig.NetRpcConfig field.
type NetRpcConfig struct {
	// EnableKeepalive is used to do a period keep alive
	// messages using a ping.
	EnableKeepAlive bool

	// KeepAliveInterval is how often to perform the keep alive
	KeepAliveInterval time.Duration

	// ConnectionWriteTimeout is meant to be a "safety valve" timeout after
	// we which will suspect a problem with the underlying connection and
	// close it. This is only applied to writes, where's there's generally
	// an expectation that things will move along quickly.
	ConnectionWriteTimeout time.Duration
}

Then I'll have the rpc_client.go pay attention to this structure to create a yamux.Config struct.