The config modules now do all the required validation, including Thing names,
ensuring we expand classes into full `ThingConfig` objects, and supplying
default values for args, kwargs, etc.

I've exposed ThingConfig and ThingServerConfig at module level as they are
likely useful in a number of scenarios, particularly writing tests.

It is possible to supply a dict whenever a ThingConfig is required - I'll include
this in the tests, but I don't want to recommend it - it's better to use the model
as it makes it harder to miss fields.

There was duplicated validation code in ThingServer, which I've removed, in favour of using the ThingServerConfig model.

I've also split up the code adding Things, so now we:
1. Create Things (and supply a ThingServerInterface)
2. Make connections between Things.
3. Add Things to the API.

Step (3) may move out of __init__at some point in the future.

If the server is started from the command line, we print out any validation errors, then exit: a stack
trace is not helpful. This also fixes an issue where ValidationError would not serialise properly,
which stopped the test code working (it used multiprocessing).

I've removed ThingServer.add_thing in favour of passing a dictionary to ThingServer.__init__
and the test code is updated to reflect that.

As a rule, when thing instances were required, I would:
1. Create the server
2. Assign `my_thing = server.things["my_thing"]`
3. Assert `assert isinstance(my_thing, MyThing)`
That was enough for `pyright` (and I guess mypy) to correctly infer the type in my test code. It's slightly more verbose, but means we can use the same code in test and production, rather than
needing a different way to create/add things in test code.

The name `thing_connection` confused people. `thing_slot` suggests we are marking the attribute and its value will be supplied later, which is exactly right. This commit makes that swap.

I've searched/replaced every instance of ThingConnection and thing_connection: we should check the docs build OK, this is perhaps most easily done in CI.

This has been removed as its functionality is provided by `pydantic.ImportString`.

It is no longer used for the fallback server: we gain nothing from the dynamic import.
In the future, if we want to make it configurable, we could use `ImportString` there too.

Now that it's imported statically, `mypy` spots typing errors with the fallback server.
These are now annotated correctly.

I've reworked core concepts and removed DirectThingClient, replacing it with a new "structure" page that I think preserves most of the still-useful content.