[S3] Add logs

[moxarth-elastic]

Part Of #2299

Adding more logs in S3 connector.

Log file: https://drive.google.com/file/d/1C_CLleP87TEcWfnNiAJvMG_gS5qdyBbx/view?usp=drive_link

Checklists

Pre-Review Checklist

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

[artem-shelkovnikov]

I've read the logs, some feedback just by reading:

1. "Successfully connected to AWS." line should be DEBUG, but failing to connect to AWS should be a warning with call to action (check credentials, check connectivity)
2. "Fetching buckets from S3" should be DEBUG
3. Need to add list of retrieved buckets in DEBUG mode
4. When fetching content of each bucket, need to add a line that says that we're fetching files from this bucket. If you can log number of records in each bucket, that will be even more awesome

[moxarth-elastic]

buildkite test this