feat: Update UTP to 1.4 and add ability to set UTP's MTU

[simon-lemay-unity]

Update UTP to 1.4. Since that version (and the newly-released 2.1) support setting the MTU at the transport level, add an API to configure that. The parameter can only be changed programmatically (it's not serialized) because we don't want users randomly messing with this value. It's really only meant for users that know what they're doing. For example we've had a few users report issues in restricted network environments where the MTU is abnormally low.

Changelog

• Added: Added MTU property to UnityTransport, which controls the low-level maximum datagram size that the transport will send. Unlike the MTU-related methods described below, this low-level MTU also applies to the individual chunks of a fragmented message. Typically there should be no need to change it unless you know your application is operating in an environment with an abnormally low MTU.
• Changed: Updated dependency on com.unity.transport to version 1.4.0.

Testing and Documentation

• No tests have been added.
• No documentation changes or additions were necessary.

[ShadauxCat]

NetworkManager has a MaximumTransmissionUnit property which controls how big a single batch of messages can be before it gets passed to the transport. Should that be tied into this value in some way?

NetworkManager also has the ability to set this value per-peer based on MTU negotiation. Should UTP also support that?

[simon-lemay-unity]

NetworkManager has a MaximumTransmissionUnit property which controls how big a single batch of messages can be before it gets passed to the transport. Should that be tied into this value in some way?

I hadn't noticed this. Should be pretty simple to use that instead of introducing a different knob in the transport itself.

NetworkManager also has the ability to set this value per-peer based on MTU negotiation. Should UTP also support that?

UTP itself doesn't have per-peer MTU. It might be possible to somewhat mimic the functionality in the UnityTransport wrapper, but there are cases where we won't be able to. For example, fragmented unreliable messages are split in MTU-sized chunks. But since that happens inside UTP, we can't do anything about it in the wrapper. I think for now the simplest solution is to have UnityTransport honor the MaximumTransmissionUnitSize value from NetworkManager, but not the per-peer MTUs.

[simon-lemay-unity]

Actually was easier than I thought to add support for peer MTUs in the wrapper, so I did that. It only works for reliable sends, but I think that's okay because that's the majority of NGO traffic. Plus for unreliable sends, the peer MTU support in NGO itself should ensure no messages larger than the MTU get to the transport.