Configuration defaults do not match serde

[BrynCooke]

When writing configuration structures it is essential that serde defaults agree with the rust Default implementation.

Failure to do this means that users will get very unexpected results depending on where their configuration stops and defaulting starts.

This isn't going to be fixed in serde serde-rs/serde#2622, so it is on us to ensure that things match.

To complete this we need:

• A new macro that will implement Default correctly for structs that dreive Deserialize.
• A test that we don't accidentally miss any structs and don't let any creep in in future.

OR:

• Ban the use field level serde defaults that specify a method. Serde will itself create an instance of the default implementation and use the values from that. This may do extra work that we don't need on expensive objects, but is arguably cleaner.
• A test that we don't accidentally miss any structs and don't let any creep in in future.

This could be seen as a serious security/stability bug, as users will not be able to rely on the default in the documentation actually taking place.

[BrynCooke]

We're going to have to go the custom macro route. The issue is that the serde auto defaulting mechanism will only work where all fields are optional.