feat: move step-ca into a plugin (C4)

plugins/step-ca/CHANGELOG.md

@@ -0,0 +1,16 @@
+# Changelog
+
+## 1.0.0
+
+Bundled with Termix 2.9.0.
+
+- Step CA sign-in moved out of core. The callback is now
+  `/plugin-api/step-ca/callback`; the 2.8 URI `/host/step-ca-callback`
+  still works and upgraded installs keep sending it until an admin turns
+  off "Use the old redirect URI".
+- The CA URL, fingerprint, provisioner and private endpoint list moved from
+  Admin Settings into this plugin's settings.
+- Issued certificates are stored encrypted in the plugin's own table, so
+  they survive a restart. Certificates cached by 2.8 are not carried over;
+  the next connection asks for one sign-in.
+- The terminal shows the sign-in dialog for Step CA hosts again.