Fix memory leak when UnsubscribePacket is reused

[sfod]

Issue #, if available:

Description of changes:

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sfod]

Caching ephemeral data on the packet right before we submit to the native client feels broken.

Unfortunately, the existing API of the UnsubscribePacket doesn't give much options on how to fix this issue:

bool initializeRawOptions(aws_mqtt5_packet_unsubscribe_view &raw_options) noexcept;

I wanted to replace this method with the following:

// Cleans up unsubscribeView on destruction.
class AwsMqtt5PacketUnsubscribeView_WrapperRAII {
    // ctor and dtor with RAII

    aws_mqtt5_packet_unsubscribe_view unsubscribeView;
};

AwsMqtt5PacketUnsubscribeView_WrapperRAII UnsubscribePacket::getRawOptions() noexcept;

, then the calling code would look like this:

bool Mqtt5ClientCore::Unsubscribe()
{
    ... 
    auto unsubscribeViewWrapper = unsubscribePacket->getRawOptions();
    int result = aws_mqtt5_client_unsubscribe(m_client, &unsubscribeViewWrapper.unsubscribeView, &options);
    return result;
}

But it'll be backward-incompatible change for UnsubscribePacket.

[bretambrose]

Why not ditch (ignore initializeFromRawOptions) and use something new instead?

[sfod]

initializeFromRawOptions has to remain valid, and it depends on the "cached" data inside the UnsubscribePacket, so we can't remove these fields, right? Having two independent methods performing the same logic by two different ways feels wrong.