Add cert-sha256 option and bump slipstream-rust

[dalisyron]

Background / motivation

The plugin currently only supports a PEM path for cert pinning. On Android 10+ scoped storage, the Shadowsocks host process can’t read Download paths without SAF, so the PEM‑path flow often breaks. slipstream‑rust now supports cert-sha256 pinning, which preserves the single‑step ss:// import workflow.

What this changes

• Adds a UI field for Pinned certificate SHA‑256 (DER hex) and passes it through plugin options (cert-sha256).
• Bumps the slipstream‑rust submodule to include cert‑sha256 support.

Related

• Depends on Add cert-sha256 pinning and ss:// helper slipstream-rust#49 (adds cert‑sha256 support + helper).

Testing

• Built a debug APK locally with Gradle (assembleDebug).

[Mygod]

See Mygod/slipstream-rust#49 (comment)