Make Container hooks more useful for dynamic registration

[alassek]

This is a (long-overdue) followup PR to our conversation related to integrating APM into a Dry-System app.

Also relates to hanami/hanami#1360

Use Case: I want to be able to dynamically decorate keys as they are added into the container system in order to perform APM metrics without having to touch every file.

Use Case: I want to audit the usage of particular namespaces in the Container using Dry-Monitor decoration, such as producing an audit log of Authorization checks.

In these use-cases, it's important that I want to react to container registration without having knowledge of precisely what keys are being registered. The decorate feature works well when keys are known in advance, but otherwise you have to enumerate the keys in the container after registration.

Enumeration is a problem, because after_finalize is called after the container is frozen, which is too late to decorate key values.

This PR makes two changes:

1. Establish an after_register hook that fires whenever a key is registered
2. Move after_finalize to happen prior to freezing the container.

This allows us to either decorate keys dynamically as they are registered, and to enumerate the container keys before the container is frozen in case we want to make alterations.

Due to the complexity of Container value storage between memoization and callable, I opted not to expose the key value directly; the hook code may choose to call resolve if it needs that.