chore: add deployment config option to append custom csp directives (#15596)

Allows adding custom static CSP directives to Coder. Niche use case but
makes this easier then creating a reverse proxy that has to replace the
header. We want to preserve our directives, so having an append option
is preferred to a "replace" option via a reverse proxy.


Closes https://github.com/coder/coder/issues/15118

coderd/apidoc/docs.go

@@ -10537,6 +10537,12 @@ const docTemplate = `{
                 "access_url": {
                     "$ref": "#/definitions/serpent.URL"
                 },
+                "additional_csp_policy": {
+                    "type": "array",
+                    "items": {
+                        "type": "string"
+                    }
+                },
                 "address": {
                     "description": "DEPRECATED: Use HTTPAddress or TLS.Address instead.",
                     "allOf": [