allow authentication strategy overrides for increased extensibility

[adamsachs]

Is your feature request related to a specific problem?

#562 describes a general effort/approach to making fidesops factories more extensible, so that developers outside of the core fidesops codebase can develop their own implementations that plug into the core factory.

we decided to break up that issue into sub-issues for each specific factory that we'd like to refactor (pun not intended). this issue covers the authentication strategy factory.

this will also, in effect, give us a mechanism for providing override escape hatches for SaaS auth mechanisms, just as we did for SaaS request execution in #815.

Describe the solution you'd like

• i think we can follow a similar approach to what was done in refactor masking strategy factory for improved extensibility #560 for the masking strategy factory. custom implementations can register themselves using a decorator, and no updates need to be made to the core fidesops codebase.
• these "custom" authentication implementations will register themselves with a given name/ID, and that will need to be referenced in the client_config.authentication.strategy field of the SaaS config. The implementations will be able to define their own configuration class/model, similar to how it works with the masking strategy factory in refactor masking strategy factory for improved extensibility #560

Additional context

[adriaaaa]

Adam will let us know if we should keep this ticket separate from the refactoring factories ticket by the end of the day today.

Other info:
this is a N2H, we can leverage our current pattern for now.

[adamsachs]

closing because the functionality will be covered as part of #562, which refactors all strategy factories into a single unified factory that allows for improved extensibility.