Implement API-level hooks

[tcarrio]

There are no global hooks nor way to add hooks to the global API client.

This is required as per the spec.

Definition of done:

• hooks can be added to the global API, and are run with flag evaluation
• associated tests

[jamescarr]

Looks like Hook support were implemented in #6?

[tcarrio]

Hooks as a general feature are available at the client level, but the singleton API should also be able to define hooks essentially globally.

This is referenced in the docs here and spec here