Use new AuthenticationStrategyInterface

[szh]

Depends on cyberark/conjur-api-python#22

Desired Outcome

Use new AuthenticationStrategy interface created in cyberark/conjur-api-python#20. This lays the groundwork for adding LDAP authentication support.

Implemented Changes

• Pass instance of AuthenticationStrategyInterface to Client constructor
• Add optional authn_type field to .conjurrc (defaults to authn)

Connected Issue/Story

CyberArk internal issue link: ONYX-20429

Changelog

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Behavior

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Security

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[szh]

@cyberark/community-and-integrations-team Can we please get a review on this so we can move forward on #411?

[rpothier]

LGTM!