A provider declares what it serves: mail's domain, the identity provider's issuer (issue 173)

Consumers read `${bound:smtp:domain}` and `${bound:oidc-client:issuer}`, and both keys reached
them only because a module's settings were laid over everything it served. Issue 173 stops that: a
setting overrides a key a served fact declares and adds none. So the two providers declare the keys
their consumers read, as the operator's value (`${setting:…}`, ADR 0155), and the setting that
already carries each fills it. Nothing a consumer reads changes.

Merges first: under the controller that still merges settings over served facts this is the same
value, and the controller that stops merging (mesh-controller, feat/the-mesh-places-its-own-files)
needs these declared before it rolls out.

modules/keycloak/module.json

@@ -51,7 +51,8 @@
     "oidc-client": {
       "authorization-path": "/protocol/openid-connect/auth",
       "token-path": "/protocol/openid-connect/token",
-      "userinfo-path": "/protocol/openid-connect/userinfo"
+      "userinfo-path": "/protocol/openid-connect/userinfo",
+      "issuer": "${setting:issuer}"
     }
   },
   "receives": {