Refactor tenants to contain tenant_id attribute similar to clients

[mikelax]

Feature Overview

Currently the clients object contains a client_id attribute that serves as a unique id for a given Client, though it is not the database primary key. There is a proposal in #16 to consider the client_id as a "url slug", in terms of the string pattern allowed.
The pattern proposed is: ^[a-z0-9-_]*$

For tenants, we currently only have a name attribute, and this can currently be any string. I would like to introduce a new attribute called tenant_id with the same string pattern enforcement. The code should then be refactored to use this id instead of the current implementation that requires but the name and iss_claim.
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