Minor fixes for agent component

[artem-shelkovnikov]

Part of https://github.com/elastic/search-team/issues/7982

Closes https://github.com/elastic/search-team/issues/8211

Bundles two changes:

• Makes it possible to run the component with fleet. This is achieved by base64 encoding the api key if it's received in a format "a:b"
• Set service._use_native_connector_api_keys to False to make the service run without using native api keys and use agent/fleet api key instead

Checklists

Pre-Review Checklist

• [x] this PR does NOT contain credentials of any kind, such as API keys or username/passwords (double check config.yml.example)
• [x] this PR has a meaningful title
• [x] this PR links to all relevant github issues that it fixes or partially addresses
• [x] if there is no GH issue, please create it. Each PR should have a link to an issue
• [x] this PR has a thorough description
• [x] Covered the changes with automated tests
• [x] Tested the changes locally
• [x] Added a label for each target release version (example: v7.13.2, v7.14.0, v8.0.0)

[github-actions[bot]]

💚 Backport PR(s) successfully created

Status	Branch	Result
✅	8.x	https://github.com/elastic/connectors/pull/2840

This backport PR will be merged automatically after passing CI.