Migrate configkit

[jmalloc]

What change does this introduce?

This PR introduces the config package as a replacement for dogmatiq/configkit.

It's not a direct replacement, as the API has changed somewhat, but it is structurally very similar and quite easy to migrate.

Why make this change?

In general it's an effort to reduce the dependency complexity for engines and related packages.

As regards the changes to the API, they are designed to address several shortcomings with configkit:

• Its inability to represent incomplete and invalid configurations, which prevented us from using the existing static analysis tools in linting / LSP applications
• Its divergence from the structure of Dogma itself. Over the years Dogma's API has changed, but configkit had not beed updated to match, leading to its inability to represent things like "routing options" and "handler registration options"
• Its "message role" centric design, which is a concept that is no longer relevant now that Command, Event and Timeout interfaces are distinct.

Is there anything you are unsure about?

No

What issues does this relate to?

#2

[jmalloc]

This is nearing completion and is being used successfully in a branch of testkit. I need to add further tests.

In general, configkit didn't make its way into the public API of the engines, but there are a few exceptions so there will be some minor BC breaks to testkit/engine (which shouldn't affect anyone). I don't think there will be any public-facing changes to verity.

[jmalloc]

I've requested a review, but I don't really expect you to go through this with a fine-toothed comb. Danil, I believe next steps will be for me to start migrating the static analysis code.

[danilvpetrov]

I like the term Fidelity for sure 👍