docs: Add axum-csrf-sync-pattern to ECOSYSTEM.md

[LeoniePhiline]

The linked crate provides a Cross-Site Request Forgery protection layer and middleware for use with the axum web framework.

The middleware implements the CSRF Synchronizer Token Pattern for AJAX backends and API endpoints as described in the OWASP CSRF prevention cheat sheet.

Motivation

Add axum-csrf-sync-pattern to ECOSYSTEM.md, to indicate its existence to other axum users who might have a need for such middleware.

Solution

This PR patches ECOSYSTEM.md, adding the CSRF Synchronizer Token Pattern middleware crate.

[davidpdrsn]

Thanks!