feat: allow signing git commits from scaffolder

adds support to sign git commits with configured PGP key in scaffolder
actions. configuration can be done either by integration or by using the
default signing key in the scaffolder config. note that this
pgp-plugin is used for signing and that it is limited to using RSA keys
and signatures made with SHA1 hashing algorithm.

this change does not support github commit signing as it cannot be done
the same way due to GitHub app and REST API being used.

closes #25934

refers to #26333

Signed-off-by: Heikki Hellgren <heikki.hellgren@op.fi>

plugins/scaffolder-node/package.json

@@ -60,6 +60,7 @@
     "@backstage/integration": "workspace:^",
     "@backstage/plugin-scaffolder-common": "workspace:^",
     "@backstage/types": "workspace:^",
+    "@isomorphic-git/pgp-plugin": "^0.0.7",
     "concat-stream": "^2.0.0",
     "fs-extra": "^11.2.0",
     "globby": "^11.0.0",