What does this PR do?

This PR fixes a type incompatibility in Hyperf\Nats\EncodedConnection when an encoder decodes a message body into a non-string value.

Currently, EncodedConnection::subscribe() and queueSubscribe() pass the decoded value directly into Message::setBody(). This works only when the decoded payload is a string. For JSON objects, arrays, YAML structures, serialized arrays, or other non-string decoded values, this causes a TypeError, because Message::setBody() expects a string.

This PR keeps the existing Message::getBody(): string and Message::setBody(string $body) API unchanged for backward compatibility, and introduces a separate decoded payload API:

$message->getBody();    // raw/backward-compatible string body
$message->getPayload(); // decoded payload
$message->hasPayload(); // whether decoded payload was set

For backward compatibility, when the decoded payload is a string, the message body is still updated with that string. When the decoded payload is not a string, the original body remains unchanged and the decoded value is available through getPayload().

This avoids changing the public body type to mixed, while fixing decoded array/object payloads.

Fixes #5222.