Add the support for custom serialization

[orchestr7]

class CustomSerializerTest {
    object ColorAsStringSerializer : KSerializer<Color> {
        override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("Color", PrimitiveKind.STRING)

        override fun serialize(encoder: Encoder, value: Color) {
            TODO()
        }

        override fun deserialize(decoder: Decoder): Color {
            val value = decoder.decodeString()
            return Color(value.toLong())
        }
    }

    @Serializable(with = ColorAsStringSerializer::class)
    data class Color(val rgb: Long)

    @Serializable
    data class Settings(val background: Color, val foreground: Color)

    @Test
    fun testDecodingWithCustomSerializer() {
        println(Toml.decodeFromString<Settings>(
            """
                [background]
                    rgb = 0
                [foreground]
                    rgb = 0
            """.trimIndent()
        ))
    }
}

will cause

This kind of node should not be processed in TomlDecoder.decodeValue(): [background]
com.akuleshov7.ktoml.exceptions.InternalDecodingException: This kind of node should not be processed in TomlDecoder.decodeValue(): [background]
	at app//com.akuleshov7.ktoml.decoders.TomlMainDecoder.decodeKeyValue$ktoml_core(TomlMainDecoder.kt:84)
	at app//com.akuleshov7.ktoml.decoders.TomlAbstractDecoder.decodeString(TomlAbstractDecoder.kt:101)
	at app//com.akuleshov7.ktoml.decoders.CustomSerializerTest$ColorAsStringSerializer.deserialize(CustomSerializerTest.kt:23)
	at app//com.akuleshov7.ktoml.decoders.CustomSerializerTest$ColorAsStringSerializer.deserialize(CustomSerializerTest.kt:15)
	at app//kotlinx.serialization.encoding.Decoder$DefaultImpls.decodeSerializableValue(Decoding.kt:260)
	at app//kotlinx.serialization.encoding.AbstractDecoder.decodeSerializableValue(AbstractDecoder.kt:16)