OIDC fixes

[egvili]

Desired Outcome

• init.py add OIDC
• Client::authenticate should declare to return expiration as well
• api_token_expiration is saved as string in CredentialData but compared with dateTime in _is_authenticated(creds)
• authn_authentication_strategy::_is_authenticated() and calculate_token_expiration() appears 4 times
• authn_authentication_strategy::authenticate() cast doesn’t work

Implemented Changes

Describe how the desired outcome above has been achieved with this PR. In particular, consider:

• What's changed? Why were these changes made?
• How should the reviewer approach this PR, especially if manual tests are required?
• Are there relevant screenshots you can add to the PR description?

Connected Issue/Story

Resolves #[relevant GitHub issue(s), e.g. 76]

CyberArk internal issue link: [insert issue ID]()

Definition of Done

At least 1 todo must be completed in the sections below for the PR to be merged.

Changelog

• [ ] The CHANGELOG has been updated, or
• [ ] This PR does not include user-facing changes and doesn't require a CHANGELOG update

Test coverage

• [ ] 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

• [ ] 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
• [ ] These changes are part of a larger initiative that will be reviewed later, or
• [ ] No behavior was changed with this PR

Security

• [ ] Security architect has reviewed the changes in this PR,
• [ ] These changes are part of a larger initiative with a separate security review, or
• [ ] There are no security aspects to these changes