fix: add missing process field and Context import

src/digest_engine.rs

@@ -1,4 +1,4 @@
-use anyhow::{anyhow, Result};
+use anyhow::{anyhow, Context, Result};
 use std::io;
 
 use lazy_static::lazy_static;

src/digest_engine/program.rs

@@ -15,6 +15,7 @@ pub struct DigestEngineProgram {
     pub program: String,
     pub args: Vec<String>,
     pub supported: bool,
+    process: Option<std::process::Child>,
 }
 
 impl DigestEngineProgram {