chore(writer): enable native writer by default (#14952)

## Description

Enables the native writer by default. The native writer can still be
disabled through `_DD_TRACE_WRITER_NATIVE`.

## Testing

All tests are run with the native writer. The native writer have also
been enabled on some services internally for dogfooding purposes.

## Risks

This changes the logic behind trace export and client-side stats
computation (for customers who enabled it). We expect no change in
behavior for trace exporting. Client-side stats behavior may show some
differences as the current python implementation is not fully compliant
with the client-side stats spec.

## Additional Notes

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Author: VianneyRuhlmann