feat: add webhook env var decoder

[lfleischmann]

Description

The webhook.hooks config option is an array of objects. When using environment variables, specifying the hooks such that the envconfig package can correctly map the values into the config struct does not seem to be possible with the built-in capabilities.

Implementation

envconfig allows defining custom decoders, though. This PR introduces an implementation so that hooks can be defined via environment variables as follows:

WEBHOOKS_HOOKS={"callback":"http://app.com/usercb","events":["user"]};{"callback":"http://app.com/emailcb","events":["email.send"]}