debugger: Mark DebugAdapterBinary::program as optional (#32534)

This allows us to support debugging with a debug adapter not managed by
Zed. Note that this is not a user facing change, as DebugAdapterBinary
is used to determine how to spawn a debugger. Thus, this should not
break any configs or anything like that.

Closes #ISSUE

Release Notes:

- N/A

crates/extension_api/wit/since_v0.6.0/dap.wit

@@ -50,7 +50,7 @@ interface dap {
     }
 
     record debug-adapter-binary {
-        command: string,
+        command: option<string>,
         arguments: list<string>,
         envs: env-vars,
         cwd: option<string>,