AddSchemaFieldsUpdateProcessorFactory does not check if the ConfigSet is immutable

SOLR-7742 introduced Immutable ConfigSets. There are checks added to SolrConfigHandler and SchemaHandler so that if a user tries to modify the SolrConfig or the Schema via either of these interfaces an error is returned if the ConfigSet is defined to be immutable.

Updates to the schema made via the AddSchemaFieldsUpdateProcessorFactory are not checked in this way. I'm not certain this should be considered a bug. A ConfigSet is defined by {SolrConfig, Schema, ConfigSetProperties}. On one hand, you can argue that you are modifying the Schema, which is part of the ConfigSet, so the immutable check should apply. On the other hand, the SolrConfig (which defines the AddSchema...Factory} defines that it wants the Config to be updated, so if you view the ConfigSet in totality you could argue nothing is really changing. I'd slightly lean towards adding the check, but could go either way.

Other opinions?