feat: Add support for NIP-55 (#18)

Reviewed-on: https://git.reya.su/reya/coop-mobile/pulls/18

composeApp/src/androidMain/kotlin/su/reya/coop/ExternalSignerLauncher.kt

@@ -0,0 +1,28 @@
+package su.reya.coop
+
+import android.content.Intent
+import androidx.activity.result.ActivityResult
+import androidx.activity.result.ActivityResultLauncher
+import kotlinx.coroutines.CompletableDeferred
+
+class ExternalSignerLauncher {
+    private var launcher: ActivityResultLauncher<Intent>? = null
+    private var pendingResult: CompletableDeferred<ActivityResult>? = null
+
+    fun register(launcher: ActivityResultLauncher<Intent>) {
+        this.launcher = launcher
+    }
+
+    suspend fun launch(intent: Intent): ActivityResult {
+        val deferred = CompletableDeferred<ActivityResult>()
+        pendingResult = deferred
+        launcher?.launch(intent)
+            ?: throw IllegalStateException("ExternalSignerLauncher not registered")
+        return deferred.await()
+    }
+
+    fun onResult(result: ActivityResult) {
+        pendingResult?.complete(result)
+        pendingResult = null
+    }
+}