Send request body for http client integrations and similar

[adinauer]

Different SDKs have different HTTP library integrations.
We plan to generate events directly from these integrations when we detect failed HTTP response status code.

• Capture event for HTTP requests resulted in server error #38

Additionally, we can attach the request and/or response bodies. This feature exists in some for in some SDKs, we'd like to discuss in a broader SDK group and define details and write the develop docs.

• Check which integrations already do it. Add checklist with integrations with their status.
• Amend develop docs. We have the current develop docs direction for HTTP library integrations
  • Maybe it should be done via attachments?
  • it doesn't say anything about Request or Response body.
  • It currently has no note regarding PII.
  • Add docs on how to deal with gotchas, how to strip data in callbacks etc.
• Public facing docs should be written under common and opt'ted-in in platforms that support it as we go.

SDKs:

• Dart