Add code to extract the base domain from a hostname

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

@@ -0,0 +1,10 @@
+package app.k9mail.autodiscovery.autoconfig
+
+/**
+ * Extract the base domain from a host name.
+ *
+ * An implementation needs to respect the [Public Suffix List](https://publicsuffix.org/).
+ */
+interface BaseDomainExtractor {
+    fun extractBaseDomain(domain: String): String
+}

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

@@ -0,0 +1,16 @@
+package app.k9mail.autodiscovery.autoconfig
+
+import okhttp3.HttpUrl
+
+class OkHttpBaseDomainExtractor : BaseDomainExtractor {
+    override fun extractBaseDomain(domain: String): String {
+        return domain.toHttpUrlOrNull().topPrivateDomain() ?: domain
+    }
+
+    private fun String.toHttpUrlOrNull(): HttpUrl {
+        return HttpUrl.Builder()
+            .scheme("https")
+            .host(this)
+            .build()
+    }
+}

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

@@ -0,0 +1,29 @@
+package app.k9mail.autodiscovery.autoconfig
+
+import assertk.assertThat
+import assertk.assertions.isEqualTo
+import org.junit.Test
+
+class OkHttpBaseDomainExtractorTest {
+    private val baseDomainExtractor = OkHttpBaseDomainExtractor()
+
+    @Test
+    fun `basic domain`() {
+        assertThat(baseDomainExtractor.extractBaseDomain("domain.example")).isEqualTo("domain.example")
+    }
+
+    @Test
+    fun `basic subdomain`() {
+        assertThat(baseDomainExtractor.extractBaseDomain("subdomain.domain.example")).isEqualTo("domain.example")
+    }
+
+    @Test
+    fun `domain with public suffix`() {
+        assertThat(baseDomainExtractor.extractBaseDomain("example.co.uk")).isEqualTo("example.co.uk")
+    }
+
+    @Test
+    fun `subdomain with public suffix`() {
+        assertThat(baseDomainExtractor.extractBaseDomain("subdomain.example.co.uk")).isEqualTo("example.co.uk")
+    }
+}