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

docs/reference/api/general.md

@@ -139,6 +139,7 @@ curl -X GET http://coder-server:8080/api/v2/deployment/config \
 			"scheme": "string",
 			"user": {}
 		},
+		"additional_csp_policy": ["string"],
 		"address": {
 			"host": "string",
 			"port": "string"