Updatable client config (#167)

[MarcusLongmuir]

Resolves #167.

This PR:

• Makes the part of the configuration of the Networked3dWebExperienceClient updatable. The config that is updatable is defined by the UpdatableConfig type.
• Breaking change - The skyboxHdrJpgUrl is replaced by environmentConfig.hdrJpgUrl and environmentConfig.hdrUrl
• Breaking change - The mmlDocuments config is now an object with keys that allow updates to the config to ensure that existing documents are diffed correctly (by their consistent key from old -> new config).
• Broadcast sending and receiving have been added to the UserNetworkingServer and UserNetworkingClient classes to allow sending arbitrary messages from the server to the connected clients (e.g. to allow sending a config update from the server to the client).

An example that sends a broadcast to connected clients can be found on the feature/update-world-config-example branch.

What kind of change does your PR introduce? (check at least one)

• Feature

Does your PR fulfill the following requirements?

• The title references the corresponding issue # (if relevant)