Add config to prevent changes to custom profile keys

**Description:**

https://github.com/element-hq/synapse/pull/17488 Added support for custom key:value pairs for user profiles, however there doesn't seem to be a way to prevent users from setting these values like with the [enable_set_displayname](https://element-hq.github.io/synapse/develop/usage/configuration/config_documentation.html#enable_set_displayname) config option

The config could either be a blanket forbid on all fields or a list of forbidden field names. I think the same behavior like with displayname and avatar_url where only admins can set the values would make sense