feat(bench-common): add certificate generation utilities

- CaCertificate: self-signed CA that can sign server certificates
- ServerCertificate: DER-encoded cert chain and private key
- generate_test_certs(): convenience helper for local testing
- Uses rcgen 0.14 for X.509 certificate generation

bench-common/Cargo.toml

@@ -5,6 +5,7 @@ authors.workspace = true
 edition.workspace = true
 
 [dependencies]
+rcgen.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 strum.workspace = true