storage: support multiple SSL CA certificates (#11341)

## Problem
- We need to support multiple SSL CA certificates for graceful root CA
certificate rotation.
- Closes: https://github.com/neondatabase/cloud/issues/25971

## Summary of changes
- Parses `ssl_ca_file` as a pem bundle, which may contain multiple
certificates. Single pem cert is a valid pem bundle, so the change is
backward compatible.

pageserver/src/controller_upcall_client.rs

@@ -76,7 +76,7 @@ impl StorageControllerUpcallClient {
             client = client.default_headers(headers);
         }
 
-        if let Some(ssl_ca_cert) = &conf.ssl_ca_cert {
+        for ssl_ca_cert in &conf.ssl_ca_certs {
             client = client.add_root_certificate(ssl_ca_cert.clone());
         }