FFI Support (#18)

This PR adds FFI (Foreign Function Interface) support to gst-plugin-edgeimpulse, enabling direct integration with the Edge Impulse C++ SDK via the `edge-impulse-ffi-rs` crate. This brings significant improvements to build flexibility, performance, and deployment options for GStreamer-based AI inference pipelines.

# Key Features
## FFI Mode (Default):
* Direct, statically-linked inference using the Edge Impulse C++ SDK.
* Models are compiled into the binary at build time for faster startup and inference.
* No need to provide a .eim model file at runtime.
* Supports all advanced hardware accelerators and backends available in the FFI SDK.
* Controlled via environment variables (`EI_PROJECT_ID`, `EI_API_KEY`, or `EI_MODEL`).
## EIM Mode (Legacy):
* Continues to support .eim model files for maximum compatibility.
* Selectable via Cargo features (--no-default-features --features eim).

# Why FFI Support?
* Performance: Direct C++ SDK integration reduces inference latency and startup time.
* Flexibility: Supports a wider range of hardware accelerators and deployment targets.
* Simplicity: No need to manage .eim files at runtime in FFI mode.
* Future-Proof: Aligns with the latest Edge Impulse SDK capabilities and deployment best practices.
# Migration Notes
* Default Mode: The plugin now defaults to FFI mode. To use EIM mode, build with --no-default-features --features eim.
* Environment Variables: FFI mode requires `EI_PROJECT_ID` and `EI_API_KEY` (or `EI_MODEL`) to be set at build time.
* Model Switching: When changing models, run cargo clean and clear the cargo cache to avoid stale artifacts.

Author: ferjm

.github/workflows/ci.yml

@@ -55,7 +55,7 @@ jobs:
       run: cargo fmt -- --check
 
     - name: Build
-      run: cargo build --verbose
+      run: cargo build --no-default-features --features eim --verbose
 
     - name: Run tests
-      run: cargo test --verbose
+      run: cargo test --no-default-features --features eim --verbose

.github/workflows/docs.yml

@@ -46,7 +46,7 @@ jobs:
         uses: dtolnay/rust-toolchain@stable
 
       - name: Build Documentation
-        run: cargo doc --no-deps
+        run: cargo doc --no-default-features --features eim --no-deps
 
       - name: Add index.html redirect
         run: |