Add .gitignore for proto-models (#22977)

When I run `./datafusion/proto-models/regen.sh` from repository root off
main, I'm getting a dirty git state like so:

```sh
datafusion (main)$ ./datafusion/proto-models/regen.sh
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.07s
     Running `/Users/jeffrey/.cargo_target_cache/debug/gen`
Copying datafusion/proto-models/src/datafusion.rs to datafusion/proto-models/src/generated/prost.rs
datafusion (main)$ git status
On branch main
Your branch is up to date with 'upstream/main'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        datafusion/proto-models/proto/proto_descriptor.bin datafusion/proto-models/src/datafusion.rs          datafusion/proto-models/src/datafusion.serde.rs    datafusion/proto-models/src/datafusion_common.rs

nothing added to commit but untracked files present (use "git add" to track)
```

Copying over the `.gitignore` from `proto` to fix this


https://github.com/apache/datafusion/blob/2282d23d4ff0af91463b63aa99cd793635ecef8e/datafusion/proto/.gitignore#L1-L5

Related PR:

- #21929

Author: Jefffrey

datafusion/proto-models/.gitignore

@@ -0,0 +1,5 @@
+# Files generated by regen.sh
+proto/proto_descriptor.bin
+src/datafusion.rs
+src/datafusion.serde.rs
+src/datafusion_common.rs