GitHub, Sandfly & Salesforce connectors split

[mattnowzari]

🚧 Connectors Framework Improvements🚧

Part of https://github.com/elastic/search-team/issues/10999 & https://github.com/elastic/search-team/issues/11003

This PR splits the following connector source files into directories:

• Salesforce
• GitHub
• Sandfly

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
• Covered the changes with automated tests
• Tested the changes locally

Related Pull Requests

• [Connectors Framework Improvements ] Break apart connectors source files into folders #3747