Add a way to install Slither plugins

This adds an extra option, `slither-plugins`, that lets users provide
a requirements.txt file to be installed alongside Slither. This can
be used to install Slither plugins.

Fixes: #16

action.yml

@@ -17,6 +17,8 @@ inputs:
     description: 'The path to the Slither configuration file. By default, `./slither.config.json` is used if present.'
   slither-version:
     description: 'The version of slither-analyzer to use. By default, the latest release in PyPI is used.'
+  slither-plugins:
+    description: 'A requirements.txt file to install alongside Slither. Useful to install custom plugins.'
   ignore-compile:
     description: 'Whether to ignore the compilation step when running crytic-compile and Slither.'
     default: false

entrypoint.sh

@@ -21,6 +21,7 @@ SARIFOUT="$4"
 SLITHERVER="$5"
 SLITHERARGS="$(get INPUT_SLITHER-ARGS)"
 SLITHERCONF="$(get INPUT_SLITHER-CONFIG)"
+SLITHERPLUGINS="$(get INPUT_SLITHER-PLUGINS)"
 STDOUTFILE="/tmp/slither-stdout"
 IGNORECOMPILE="$(get INPUT_IGNORE-COMPILE)"
 
@@ -185,6 +186,13 @@ install_slither()
     export PATH="/opt/slither/bin:$PATH"
     pip3 install wheel
     pip3 install "$SLITHERPKG"
+
+    if [[ -n "$SLITHERPLUGINS" ]]; then
+        echo "[-] Slither plugins provided, installing them"
+        pushd "$(dirname "$SLITHERPLUGINS")" >/dev/null
+        pip3 install -r "$(basename "$SLITHERPLUGINS")"
+        popd >/dev/null
+    fi
 }
 
 install_deps()