✨ Added close_connection() to sap_rfc

[AnnaGerlich]

Summary

Added close_connection() to ensure closing connection after each session.
This change is in line with the viadot main branch: #676

Importance

Closing connection after each session will fix error with too many open connections in SAP as there is a limit of open SAP connections.

Checklist

This PR:

• follows the guidelines laid out in CONTRIBUTING.md
• links relevant issue(s)
• adds/updates tests (if appropriate)
• adds/updates docstrings (if appropriate)
• adds an entry in CHANGELOG.md

[trymzet]

@AnnaGerlich pls create issue for contributing.md process as discussed