Make PurchasesError java.io.Serializable (#2031)

### Description
This was requested in #2006

purchases/src/androidTest/kotlin/com/revenuecat/purchases/ErrorsTest.kt

@@ -0,0 +1,39 @@
+package com.revenuecat.purchases
+
+import android.os.Bundle
+import android.os.Parcel
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.Test
+import org.junit.runner.RunWith
+
+@RunWith(AndroidJUnit4::class)
+class ErrorsTest {
+
+    @Test
+    fun errorIsSerializable() {
+        val purchasesError = PurchasesError(
+            PurchasesErrorCode.ConfigurationError,
+            "Underlying error message",
+        )
+        val purchasesException = PurchasesException(purchasesError)
+        val bundle = Bundle().apply {
+            putSerializable("exception", purchasesException)
+        }
+        val parcel = Parcel.obtain()
+        val readBundle: Bundle?
+        try {
+            // Write the bundle to a parcel in order to actually perform the serialization. Without this,
+            // the bundle will not be serialized and the test will be successful while not actually being serializable.
+            parcel.writeBundle(bundle)
+            parcel.setDataPosition(0)
+            readBundle = parcel.readBundle(javaClass.classLoader)!!
+        } finally {
+            parcel.recycle()
+        }
+        assertThat(readBundle).isNotNull
+        val serializedException = readBundle!!.getSerializable("exception", PurchasesException::class.java)
+        assertThat(serializedException!!.code).isEqualTo(purchasesException.code)
+        assertThat(serializedException.underlyingErrorMessage).isEqualTo(purchasesException.underlyingErrorMessage)
+    }
+}

purchases/src/main/kotlin/com/revenuecat/purchases/errors.kt

@@ -3,6 +3,7 @@ package com.revenuecat.purchases
 import android.os.Parcelable
 import kotlinx.parcelize.IgnoredOnParcel
 import kotlinx.parcelize.Parcelize
+import java.io.Serializable
 
 typealias PurchasesErrorCallback = (PurchasesError) -> Unit
 
@@ -16,7 +17,12 @@ typealias PurchasesErrorCallback = (PurchasesError) -> Unit
 class PurchasesError(
     val code: PurchasesErrorCode,
     val underlyingErrorMessage: String? = null,
-) : Parcelable {
+) : Parcelable, Serializable {
+
+    companion object {
+        private const val serialVersionUID = 81719171L
+    }
+
     // Message explaining the error
     @IgnoredOnParcel
     val message: String = code.description