This removes most of the code used for polylith/API mode.
This removes the `/api` internal endpoints entirely.
Binary size change roughly 5%:
```
51437560 Feb 13 10:15 dendrite-monolith-server # old
48759008 Feb 13 10:15 dendrite-monolith-server # new
```
This makes the following changes:
* The various `Defaults` functions are now responsible for setting sane defaults if `generate` is specified, rather than hiding them in `generate-config`
* Some configuration options have been marked as `omitempty` so that they don't appear in generated configs unnecessarily (monolith-specific vs. polylith-specific options)
* A new option `-polylith` has been added to `generate-config` to create a config that makes sense for polylith deployments (i.e. including the internal/external API listeners and per-component database sections)
* A new option `-normalise` has been added to `generate-config` to take an existing file and add any missing options and/or defaults
* Add stub functions for MSC2946
* Implement core space walking algorithm
* Flesh out stub functions; add test stubs
* Implement storage bits and add sanity check test
* Implement world_readable auth with test
* Linting
