Add resource exists command

[szh]

Depends on cyberark/conjur-api-python#32

Desired Outcome

Add the Resource command and exists subcommand which existed in the Ruby CLI but was not added to the Python CLI.

Implemented Changes

Added support for command with syntax conjur resource exists -i user:someuser

Connected Issue/Story

CyberArk internal issue link: 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: ONYX-22999
• 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. One small suggestion, feel free to take or leave it.