Add non-generic Find to context

[ajcvickers]

In the old stack we had a non-generic Set method on the context, which then allowed non-generic Find to be used. We decided previously against non-generic Set for good reasons, and I don't think we should change that. However, we should consider having a non-generic Find on the context. This is useful for things like generic repositories and other generic context use.

public virtual object Find(Type entityType, params object[] keyValues)

public virtual Task<object> FindAsync(Type entityType, params object[] keyValues)

public virtual Task<object> FindAsync(Type entityType, object[] keyValues, CancellationToken cancellationToken)

For completeness we could also add generic versions:

public virtual TEntity Find<TEntity>(params object[] keyValues)

public virtual Task<TEntity> FindAsync<TEntity>(params object[] keyValues)

public virtual Task<TEntity> FindAsync<TEntity>(object[] keyValues, CancellationToken cancellationToken)