Eventing function scope is silently ignored by pre-7.1 servers

The Eventing Service introduced the concept of eventing function scope in Couchbase Server 7.1. A user of the HTTP API assigns a scope to a function by including a "function_scope" field in the JSON representation when creating or updating a function.

When a version of Couchbase Server prior to 7.1 receives this JSON, it silently ignores the "function_scope" field. This results in the function being created in the admin scope instead of the scope the user specified.

In general, the SDKs throw a `FeatureNotAvailable` exception if the user attempts an operation not supported by their version of Couchbase Server. However, in this case there is no indication the function scope was ignored. This makes it impractical for an SDK to notify a user they've done something wrong.