Add Domain value class

core/common/src/main/kotlin/app/k9mail/core/common/net/Domain.kt

@@ -0,0 +1,8 @@
+package app.k9mail.core.common.net
+
+@JvmInline
+value class Domain(val value: String) {
+    init {
+        requireNotNull(HostNameUtils.isLegalHostName(value)) { "Not a valid domain name: '$value'" }
+    }
+}

core/common/src/test/kotlin/app/k9mail/core/common/net/DomainTest.kt

@@ -0,0 +1,25 @@
+package app.k9mail.core.common.net
+
+import assertk.assertFailure
+import assertk.assertThat
+import assertk.assertions.hasMessage
+import assertk.assertions.isEqualTo
+import assertk.assertions.isInstanceOf
+import kotlin.test.Test
+
+class DomainTest {
+    @Test
+    fun `valid domain`() {
+        val domain = Domain("domain.example")
+
+        assertThat(domain.value).isEqualTo("domain.example")
+    }
+
+    @Test
+    fun `invalid domain should throw`() {
+        assertFailure {
+            Domain("invalid domain")
+        }.isInstanceOf<IllegalArgumentException>()
+            .hasMessage("Not a valid domain name: 'invalid domain'")
+    }
+}