Hubspot source: add `leads` stream

[topefolorunso]

Connector Name

source-hubspot

❓ What

The Hubspot connector does not currently pull Leads, which is used for prospect analytics.

📈 Impact

Users can utilize the Leads stream from Hubspot

🤝 Proposed solution

Add a new stream leads using the endpoint: https://developers.hubspot.com/docs/api/crm/leads The properties should be split into top-level fields, similar to the implementation done for existing Hubspot streams.