Limit change tracking proxies to only changing _and_ changed strategies

[ajcvickers]

Fixes #21920

[ajcvickers]

@Pilchie RC1

[ajcvickers]

/cc @JonPSmith FYI

[AndriySvyryd]

If interfacesToProxy is empty we shouldn't create a proxy for change tracking

[ajcvickers]

Will do this in another PR

[AndriySvyryd]

Additional things to fix in the next PR:

• Currently context.CreateProxy<Dictionary<string, object>>() throws "The entity type '{entityType}' was not found. Ensure that the entity type has been added to the model.". If the type is shared or weak we should instead suggest calling context.Set<T>("TypeName").CreateProxy()
• If change tracking proxies are enabled and there are entity types using Dictionary<string, object> with non-PK properties we should throw in ProxyChangeTrackingConvention.OnModelFinalized

[AndriySvyryd]

We should also validate that if the entity type is set up to use proxies then CreateProxy is used to create the instance to be tracked. (The type of the entity should at least differ from the CLR type specified in the entity type)

[ajcvickers]

@AndriySvyryd Yes, we require all properties to be virtual, but we're missing checking indexer properties--that was one of the issues I mentioned on the call earlier. This is the only reason our tests are passing right now. No foot shooting allowed.

For lazy-loading proxies, you don't have to create a proxy instance, but for change tracking proxies we don't allow you to use a non-proxy instance. This can be a pain, but prevents some serious foot shooting.

[Pilchie]

Approved for RC1