[FEATURE] Client Manager Role

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Is your feature request related to a problem? Please describe.

An issue stopping my company from considering this project for project management is a missing user role. The closest one would be the Company Manager but it is too permissive for our use. If a user is assigned to a specific client we do not want that user to have access to view/create/edit/etc projects and other information from other clients.

Describe the solution you'd like

Adding a user role of Client Manager and allowing the same permissions as the Company Manager but restricting those permissions to just those of the client selected.

We are a B2B company and some managers are in charge of multiple clients and being able to add a user to multiple clients, as requested in #420, would also work well with this role addition.

Additional context

An example would be: A manager for a client that gets marketing planning services should not be able to see another client that gets IT services. The two employee roles involved in that example are fundamentally different and the access to information should not be shared between the two managers. The marketing manager should not know IT related details of a client's network and systems just as the IT manager does not need to know the details of marketing plans of another client.