Support for Unity WebGL builds

[jdetter]

Currently WebGL builds are not supported because SpacetimeDB uses threads to process incoming network messages. I think there are several solutions here:

1. We could use Task everywhere instead of threads - we have to determine the performance impact of doing this. It needs to be tested against BitCraft.
2. We could use #ifdef to only use threads for non-webgl platforms and then use Task instead when we are building for WebGL.

First let's just try #1 and then if that doesn't work well then let's go with #2.

Steps to move this forward

• Try getting this working with a webgl build in blackholio Add support for browser platform #274
• ^ If that works, then try getting it working with BitCraft (not in webgl, just standalone) - then determine the performance impact of this.

[rekhoff]

WebGL does not support includes for the System.Net namespaces for security reasons on browsers, which means System.Net.Sockets or System.Net.WebSockets can't be used.
I've evaluated the following solutions and determined they are incompatible with our needs:

• Unity’s transport package - WebGL is only supported if both server and client are using the Unity Transport package.
• websocket-sharp - Lacks support for Custom Headers.

Currently investigating:

• NativeWebSocket

[jdetter]

We're making good progress on this, I'm moving this to 1.1.0 so that it's easier to track.

[bfops]

Potentially related: #286

[jdetter]

Fixed by #286