chore(telemetry): add session id support (#1817)

## What does this PR do?

Adds optional DD-Session-ID, DD-Root-Session-ID, and DD-Parent-Session-ID to telemetry HTTP requests in libdd-telemetry, and exposes matching fields through libdd-telemetry-ffi (builder setters / enum properties, same pattern as runtime_id). Values are stored on Config and attached in the worker when building requests.
Payload JSON is unchanged except for the existing runtime_id field in telemetry data.

## Motivation

Store session correlation needs these headers on telemetry traffic so backends can group instances and sessions without changing payload shape. This PR implements the shared client/FFI side; SDKs still own IDs and propagation.

## How to test

```bash
cargo test -p libdd-telemetry --lib
cargo test -p libdd-telemetry-ffi --lib
```

Co-authored-by: munir.abdinur <munir.abdinur@datadoghq.com>

Author: mabdinur