fix!: merge payload and data fields of Request

[vdusek]

Description

• We had data and payload fields on the Request model.
  • payload was not being provided to the HTTP clients, only the data field.
• ~In this PR, I'm merging them together, keeping only the data field (use the same naming as HTTPX & CurlImpersonate).~
• In this PR, I'm merging them together, keeping only the payload field (use the same naming as in JS Crawlee).
• ~I also defined type aliases for HTTP data and HTTP query parameters and used them across the project.~
• ~Some struggle with Pydantic & serialization, but should be OK now...~

Issues

• Reported by @honzajavorek on Slack
• Closes #560

Testing

• The current set of unit tests should cover the changes.

Checklist

• [x] CI passed