Add resource_exists method

[szh]

Desired Outcome

Add method to check whether a resource exists. Mimics what existed in the Ruby API.

Implemented Changes

• Added a resource_exists method to the Client and Api classes which invokes the resources/{account}/{kind}/{identifier} url on the Conjur server and checks the status code for 404 vs 200/403.

Connected Issue/Story

Supports CyberArk internal issue: ONYX-23083

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

[gl-johnson]

LGTM!