Relation handler cannot handle FieldOperation values

[brettwillis]

The FieldOperation (similar to the native FieldValue) class is currently a private API, so this is low priority. But we may wish to make FieldOperation a public API, at which point this will be a problem.

If the user runs a direct transaction (model.runTransaction(...)) and uses FieldOperation to atomically update a relation attribute, then the RelationHandler will crash - it cannot yet handle FieldOperation instances.

See:

https://github.com/arrowheadapps/strapi-connector-firestore/blob/09bd0e5babf66f1dce6a3ed9b7aa074f7583e558/src/utils/relation-handler.ts#L335-L337