plugin/forward: Allow Proxy to be used outside of forward plugin. (#5951)

* plugin/forward: Move Proxy into pkg/plugin/proxy, to allow forward.Proxy to be used outside of forward plugin.

Signed-off-by: Patrick Downey <patrick.downey@dioadconsulting.com>

plugin/pkg/proxy/errors.go

@@ -0,0 +1,26 @@
+package proxy
+
+import (
+	"errors"
+)
+
+var (
+	// ErrNoHealthy means no healthy proxies left.
+	ErrNoHealthy = errors.New("no healthy proxies")
+	// ErrNoForward means no forwarder defined.
+	ErrNoForward = errors.New("no forwarder defined")
+	// ErrCachedClosed means cached connection was closed by peer.
+	ErrCachedClosed = errors.New("cached connection was closed by peer")
+)
+
+// Options holds various Options that can be set.
+type Options struct {
+	// ForceTCP use TCP protocol for upstream DNS request. Has precedence over PreferUDP flag
+	ForceTCP bool
+	// PreferUDP use UDP protocol for upstream DNS request.
+	PreferUDP bool
+	// HCRecursionDesired sets recursion desired flag for Proxy healthcheck requests
+	HCRecursionDesired bool
+	// HCDomain sets domain for Proxy healthcheck requests
+	HCDomain string
+}