Allow reference data to be pre-loaded into the context

[ajcvickers]

That is, entity instances which are read-only and should always be present without needing to query from the database.

[gojanpaolo]

Might be related to #7586 Feature request: read only entities

[AndriySvyryd]

Related to #14922

[douglasg14b]

This feature would be very welcome! It can be bothersome having to make DB round-trips for read-only data to fill in entity graphs when they could be permanently in memory.

[borzag]

In my project we have struggled a lot when we have tried to preload enum like read only data, share it among several unit of work (dbcontext instances ) and tried to have ef understand that it's existing data and it's never should be created in the database. We, as others have tried, did it to avoid ef keep track of it and to gain performance. It has driven us crazy to understand why we constantly get the error that ef/sql server could not insert the data because the key already exit. We have tried so many approaches and ways to get the changetracker understand what we want.

So as a short summary, this issue would be a VERY much appreciated feature to us in our project.