fix(api): make proto/ vendor-able (console-rs#128)

I work with a large monorepo that `cargo vendor'`s all third-party code.
We use tokio quite a bit, and want to experiment with `tokio/console`

When vendoring `console-api` the `../proto` directory is lost, and the
crate is unbuildable. I understand that keeping `proto/` as a top-level
directory is probably desirable, so this change symlinks that directory
into that sub-crate itself.

My understanding is that this change may also be required once `console`
is put on crates.io?

`cargo vendor` before this change:
```
Cargo.toml build.rs   src
```
after:
```
Cargo.toml
build.rs
proto
src
```

This is basically copying what https://github.com/dtolnay/cxx does with
symlinks

Author: guswynn

console-api/build.rs

@@ -2,14 +2,14 @@ use std::error::Error;
 
 fn main() -> Result<(), Box<dyn Error>> {
     let iface_files = &[
-        "../proto/trace.proto",
-        "../proto/common.proto",
-        "../proto/tasks.proto",
-        "../proto/instrument.proto",
-        "../proto/resources.proto",
-        "../proto/async_ops.proto",
+        "proto/trace.proto",
+        "proto/common.proto",
+        "proto/tasks.proto",
+        "proto/instrument.proto",
+        "proto/resources.proto",
+        "proto/async_ops.proto",
     ];
-    let dirs = &["../proto"];
+    let dirs = &["proto"];
 
     tonic_build::configure()
         .build_client(true)

console-api/proto

@@ -0,0 +1 @@
+../proto