fix(wasm): add backward compatibility for openssh_format attribute in wasm_keygen

When PR branches merge with main for CI testing, main's BUILD files still
use openssh_format=False in wasm_keygen rules. This attribute was removed
in commit 06d8d71 when switching to the Go wrapper approach.

Add the attribute back as deprecated/ignored to maintain backward compatibility
during the merge, preventing CI build failures.

Author: avrabe

wasm/wasm_signing.bzl

@@ -60,6 +60,10 @@ wasm_keygen = rule(
             default = "key.secret",
             doc = "Name of the secret key file to generate",
         ),
+        "openssh_format": attr.bool(
+            default = False,
+            doc = "Deprecated: Ignored. wasmsign2 keygen always generates compact format keys.",
+        ),
         "_wasmsign2_wrapper": attr.label(
             default = "//tools/wasmsign2_wrapper",
             executable = True,