Implement SSH key material loading with KeySource, load_private_key, load_public_keys, and CertAuthorityEntry

Cargo.lock

@@ -4275,6 +4275,19 @@ version = "0.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7b2093cf4c8eb1e67749a6762251bc9cd836b6fc171623bd0a9d324d37af2417"
 
+[[package]]
+name = "tempfile"
+version = "3.27.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "32497e9a4c7b38532efcdebeef879707aa9f794296a4f0244f6f69e9bc8574bd"
+dependencies = [
+ "fastrand",
+ "getrandom 0.4.2",
+ "once_cell",
+ "rustix",
+ "windows-sys 0.61.2",
+]
+
 [[package]]
 name = "thiserror"
 version = "1.0.69"
@@ -5466,6 +5479,7 @@ dependencies = [
  "russh",
  "rustls",
  "rustls-acme",
+ "tempfile",
  "thiserror 2.0.18",
  "tokio",
  "tokio-rustls",