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/clientmaker.go

@@ -0,0 +1,25 @@
+package sessionresolver
+
+import "github.com/ooni/probe-cli/v3/internal/engine/netx"
+
+// dnsclientmaker makes a new resolver.
+type dnsclientmaker interface {
+	// Make makes a new resolver.
+	Make(config netx.Config, URL string) (childResolver, error)
+}
+
+// clientmaker returns a valid dnsclientmaker
+func (r *Resolver) clientmaker() dnsclientmaker {
+	if r.dnsClientMaker != nil {
+		return r.dnsClientMaker
+	}
+	return &defaultDNSClientMaker{}
+}
+
+// defaultDNSClientMaker is the default dnsclientmaker
+type defaultDNSClientMaker struct{}
+
+// Make implements dnsclientmaker.Make.
+func (*defaultDNSClientMaker) Make(config netx.Config, URL string) (childResolver, error) {
+	return netx.NewDNSClient(config, URL)
+}