Add Encoder generics

[kafkiansky]

Generics will allow writing encoders with a more specific type than object. For example, this makes it possible to implement a RawMessageEncoder that enables using a protobuf serializer other than thesis/protobuf (e.g google/protobuf).

/**
 * @api
 */
final readonly class RawMessage
{
    public function __construct(
        public string $payload,
    ) {}
}

/**
 * @template-implements Encoder<RawMessage>
 */
final readonly class RawMessageEncoder implements Encoder
{
	#[\Override]
    public function name(): string
	{
		return 'proto';
	}

    #[\Override]
    public function encode(object $request): string
	{
		return $request->payload;
	}

    #[\Override]
    public function decode(string $buffer, string $classType): object
	{
		return new $classType($buffer);
	}
}