chore: Reorganize examples and interop crates (#180)

* chore: Reorganize examples and interop crates

* fix interop tests

examples/src/gcp/README.md

@@ -0,0 +1,21 @@
+# Google Cloud Pubsub example
+
+This example will attempt to fetch a list of topics using the google
+gRPC protobuf specification. This will use an OAuth token and TLS to
+fetch the list of topics.
+
+First, you must generate a access token via the [OAuth playground]. From here
+select the `Cloud Pub/Sub API v1` and its urls as the scope. This will start
+the OAuth flow. Then you must hit the `Exchange authorization code for tokens`
+button to generate an `access_token` which will show up in the HTTP response
+to the right under the `access_token` field in the response json.
+
+Once, you have this token you must fetch your GCP project id which can be found
+from the main page on the dashboard. When you have both of these items you can
+run the example like so:
+
+```shell
+GCP_AUTH_TOKEN="<access-token>" cargo run --bin gcp-client -- <project-id>
+```
+
+[OAuth playground]: https://developers.google.com/oauthplayground/

examples/src/gcp/client.rs

@@ -0,0 +1,55 @@
+pub mod api {
+    tonic::include_proto!("google.pubsub.v1");
+}
+
+use api::{publisher_client::PublisherClient, ListTopicsRequest};
+use http::header::HeaderValue;
+use tonic::{
+    transport::{Certificate, Channel, ClientTlsConfig},
+    Request,
+};
+
+const ENDPOINT: &str = "https://pubsub.googleapis.com";
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let token = std::env::var("GCP_AUTH_TOKEN").map_err(|_| {
+        "Pass a valid 0Auth bearer token via `GCP_AUTH_TOKEN` environment variable.".to_string()
+    })?;
+
+    let project = std::env::args()
+        .skip(1)
+        .next()
+        .ok_or("Expected a project name as the first argument.".to_string())?;
+
+    let bearer_token = format!("Bearer {}", token);
+    let header_value = HeaderValue::from_str(&bearer_token)?;
+
+    let certs = tokio::fs::read("tonic-examples/data/gcp/roots.pem").await?;
+
+    let tls_config = ClientTlsConfig::with_rustls()
+        .ca_certificate(Certificate::from_pem(certs.as_slice()))
+        .domain_name("pubsub.googleapis.com");
+
+    let channel = Channel::from_static(ENDPOINT)
+        .intercept_headers(move |headers| {
+            headers.insert("authorization", header_value.clone());
+        })
+        .tls_config(tls_config)
+        .connect()
+        .await?;
+
+    let mut service = PublisherClient::new(channel);
+
+    let response = service
+        .list_topics(Request::new(ListTopicsRequest {
+            project: format!("projects/{0}", project),
+            page_size: 10,
+            ..Default::default()
+        }))
+        .await?;
+
+    println!("RESPONSE={:?}", response);
+
+    Ok(())
+}