Update rules action schema for multiple cardinality context

Problem/Motivation

After #2648300: Support multiple contexts in the UI the rules_send_email action allows multiple ('to' field) recipients. There are only a couple of context values that have an explicit schema defined, but rules_send_email is one of them. The to context is still defined as single value in the schema, so trying to save multiple recipients throws an error.

Proposed resolution

Change the schema for 'to' from email to sequence.