Abstract authentication flow into new interface

[szh]

Note: To see implementation in the CLI, see https://github.com/cyberark/cyberark-conjur-cli/pull/410

Desired Outcome

Create a new AuthenticationStrategy interface that will allow us to abstract the authentication flow. The interface should take a CredentialProvider and Conjur account as inputs, as well as any other arguments that are needed to authenticate, such as service_id for LDAP. The interface should have a authenticate method which returns a Conjur auth token. All details of authentication should be handled by the AuthenticationStrategy interface. Include unit tests and update existing unit and integration tests.

Implemented Changes

• Added AuthenticationStrategyInterface to abstract authentication logic
• Added AuthnAuthenticationStrategy which implements the interface for the default authn strategy
• Updated the Api class to use the interface to implement authentication

Connected Issue/Story

CyberArk internal issue link: ONYX-20429

Changelog

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• [ ] This PR does not include user-facing changes and doesn't require a CHANGELOG update

Test coverage

• [x] This PR includes new unit and integration tests to go with the code changes, or
• [ ] The changes in this PR do not require tests

Documentation

• [x] Docs (e.g. READMEs) were updated in this PR
• [ ] A follow-up issue to update official docs has been filed here: [insert issue ID]()
• [ ] This PR does not require updating any documentation

Behavior

• [ ] This PR changes product behavior and has been reviewed by a PO, or
• [x] These changes are part of a larger initiative that will be reviewed later, or
• [ ] No behavior was changed with this PR

Security

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• [x] These changes are part of a larger initiative with a separate security review, or
• [ ] There are no security aspects to these changes