refactor(sessionresolver): adapt to changing network conditions (#238)

* feat(sessionresolver): try many and use what works

* fix(sessionresolver): make sure we can use quic

* fix: the config struct is unnecessary

* fix: make kvstore optional

* feat: write simple integration test

* feat: start adding tests

* feat: continue writing tests

* fix(sessionresolver): add more unit tests

* fix(sessionresolver): finish adding tests

* refactor(sessionresolver): changes after code review

internal/engine/internal/sessionresolver/errwrapper.go

@@ -0,0 +1,23 @@
+package sessionresolver
+
+import (
+	"errors"
+	"fmt"
+)
+
+// errwrapper wraps an error to include the URL of the
+// resolver that we're currently using.
+type errwrapper struct {
+	error
+	URL string
+}
+
+// Error implements error.Error.
+func (ew *errwrapper) Error() string {
+	return fmt.Sprintf("<%s> %s", ew.URL, ew.error.Error())
+}
+
+// Is allows consumers to query for the type of the underlying error.
+func (ew *errwrapper) Is(target error) bool {
+	return errors.Is(ew.error, target)
+}