feat: Make INetworkStreamDriverConstructor useful

[simon-lemay-unity]

This PR addresses MTTB-1331 by making it simpler to use a custom driver constructor without breaking things.

INetworkStreamDriverConstructor is a mechanism that was added a long time ago that allows users to override the creation of the NetworkDriver that UnityTransport is using. As the name might hint at, it was pulled directly out of N4E and was never properly integrated into UnityTransport.

For example, before this PR, a user implementing a custom driver constructor would need to recreate the settings and pipelines manually if they wanted to mimic the default behavior of UnityTransport, which required reading the code to figure out how we are settings things up. In the case of pipelines, it's not even possible to do that in all cases because one of the pipelines used when the multiplayer tools package is installed is internal.

This PR addresses this by making both the default settings and default pipeline configurations available through the new GetDefaultNetworkSettings and GetDefaultPipelineConfigurations methods. A user can use these methods to reuse or extend the configuration UnityTransport uses by default. The (new and improved) documentation of the driver constructor interface provides an example of how to do so.

As a bonus, this centralizes all the NetworkSettings in one place which makes the code a tiny bit more organized.

Changelog

• Added: Added methods GetDefaultNetworkSettings and GetDefaultPipelineConfigurations to UnityTransport. These can be used to retrieve the default settings and pipeline stages that are used by UnityTransport. This is useful when providing a custom driver constructor through UnityTransport.s_DriverConstructor, since it allows reusing or tuning the existing configuration instead of trying to recreate it. This means a transport with a custom driver can now easily benefit from most of the features of UnityTransport, like integration with the Network Simulator and Network Profiler from the multiplayer tools package.

Testing and Documentation

• No tests have been added.
• Includes edits to existing public API documentation.

Backport

I don't plan on backporting this, unless the team feels strongly otherwise.