Allow injecting certificate for tests

Built on #609

When activating the `puffin-test-custom-ca-cert` feature, you can inject a custom ssl certificate by setting `PUFFIN_TEST_CA_CERT_PEM` to a pem file, e.g.

```bash
PUFFIN_TEST_CA_CERT_PEM=$(pwd)/mitmproxy-ca-cert.pem ./offlinepi record cargo test --features pypi --features puffin-test-custom-ca-cert -- --test-threads=1
```

This feature is off by default, so this is not possible in release builds.

.gitignore

@@ -16,3 +16,6 @@ cache-*
 
 # python tmp files
 __pycache__
+
+scripts/offlinepi/mitmproxy-ca-cert.pem
+scripts/offlinepi/responses.dat

crates/puffin-client/Cargo.toml

@@ -35,3 +35,6 @@ url = { workspace = true }
 [dev-dependencies]
 anyhow = { workspace = true }
 tokio = { workspace = true, features = ["fs", "macros"] }
+
+[features]
+puffin-test-custom-ca-cert = []

crates/puffin-client/src/registry_client.rs

@@ -74,11 +74,22 @@ impl RegistryClientBuilder {
 
     pub fn build(self) -> RegistryClient {
         let client_raw = {
-            let client_core = ClientBuilder::new()
+            let mut client_core = ClientBuilder::new()
                 .user_agent("puffin")
                 .pool_max_idle_per_host(20)
                 .timeout(std::time::Duration::from_secs(60 * 5));
 
+            if cfg!(feature = "puffin-test-custom-ca-cert") {
+                if let Some(cert) = std::env::var_os("PUFFIN_TEST_CA_CERT_PEM") {
+                    client_core = client_core.add_root_certificate(
+                        reqwest::Certificate::from_pem(
+                            &fs_err::read(cert).expect("No PUFFIN_TEST_CA_CERT_PEM"),
+                        )
+                        .expect("Invalid certificate"),
+                    )
+                }
+            }
+
             client_core.build().expect("Fail to build HTTP client.")
         };