Add ability to configure exceptions that can be assigned to

[sloops77]

In certain frameworks, like fastify, it is desirable to modify certain variables in plugins due to framework architecture. These variables prove to be exceptions to the better-mutation rules.

Think something like the req.user in a fastify-auth plugin.

Each team needs to decide how they want to handle these exceptions

1. Centralized config
2. Using // eslint-disable eslint-plugin-better-mutation