Add code to do a DNS MX lookup

feature/autodiscovery/autoconfig/build.gradle.kts

@@ -8,6 +8,7 @@ dependencies {
 
     compileOnly(libs.xmlpull)
     implementation(libs.okhttp)
+    implementation(libs.minidns.hla)
 
     testImplementation(libs.kxml2)
     testImplementation(libs.jsoup)

feature/autodiscovery/autoconfig/src/main/kotlin/app/k9mail/autodiscovery/autoconfig/MiniDnsMxResolver.kt

@@ -0,0 +1,13 @@
+package app.k9mail.autodiscovery.autoconfig
+
+import org.minidns.hla.ResolverApi
+import org.minidns.record.MX
+
+class MiniDnsMxResolver : MxResolver {
+    override fun lookup(domain: String): List<String> {
+        val result = ResolverApi.INSTANCE.resolve(domain, MX::class.java)
+        return result.answersOrEmptySet
+            .sortedBy { it.priority }
+            .map { it.target.toString() }
+    }
+}

feature/autodiscovery/autoconfig/src/main/kotlin/app/k9mail/autodiscovery/autoconfig/MxResolver.kt

@@ -0,0 +1,8 @@
+package app.k9mail.autodiscovery.autoconfig
+
+/**
+ * Look up MX records for a domain.
+ */
+interface MxResolver {
+    fun lookup(domain: String): List<String>
+}

feature/autodiscovery/autoconfig/src/test/kotlin/app/k9mail/autodiscovery/autoconfig/MiniDnsMxResolverTest.kt

@@ -0,0 +1,30 @@
+package app.k9mail.autodiscovery.autoconfig
+
+import assertk.all
+import assertk.assertThat
+import assertk.assertions.containsExactlyInAnyOrder
+import assertk.assertions.index
+import assertk.assertions.isEqualTo
+import kotlin.test.Ignore
+import kotlin.test.Test
+
+class MiniDnsMxResolverTest {
+    private val resolver = MiniDnsMxResolver()
+
+    @Test
+    @Ignore("Requires internet")
+    fun `MX lookup for known domain`() {
+        val result = resolver.lookup("thunderbird.net")
+
+        assertThat(result).all {
+            index(0).isEqualTo("aspmx.l.google.com")
+            containsExactlyInAnyOrder(
+                "aspmx.l.google.com",
+                "alt1.aspmx.l.google.com",
+                "alt2.aspmx.l.google.com",
+                "alt4.aspmx.l.google.com",
+                "alt3.aspmx.l.google.com",
+            )
+        }
+    }
+}