Test skip processing headers

[fpacifici]

BUilt on top of #299

One of the main throughput issues we observed in the tests mentioned in
#299 is that headers are
really expensive to pass to and from rust. Most of the issue seems
related with https://github.com/getsentry/streams/blob/main/sentry_streams/src/messages.rs#L47-L82.
Specifically this code is executed every time python references
headers.

This is a test to confirm the impact by removing all the headers related
logic.
All messages are created with empty headers, the header fetching logic
is never called.
I left the support for headers in the rust code because the consumer
to test relies on a header filter so I cannto remove it entirely.

This change is suppsoed to be reverted after we perform a production test

[sentry]

Bug: The PyMessage and PyRawMessage constructors accept a headers parameter but silently ignore it, always initializing headers to an empty list.
_{Severity: MEDIUM}

Suggested Fix

The headers parameter should be used to initialize the self._headers attribute in the PyMessage and PyRawMessage constructors. If the intention is to permanently remove header handling, the headers parameter should be removed from the constructor signatures and all call sites to prevent silent data loss.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: sentry_streams/sentry_streams/pipeline/message.py#L110

Potential issue: The constructors for `PyMessage` and `PyRawMessage` accept a `headers`
parameter but the implementation ignores it, always initializing the internal `_headers`
attribute to an empty list. This means any headers passed when creating these message
objects are silently discarded. This violates the API contract and can lead to
unexpected behavior or silent failures in any code that relies on headers being
preserved across processing steps, such as custom filters or monitoring logic.

_{Did we get this right? 👍 / 👎 to inform future reviews.}