Add GetDbSets and fix error details of incorrect Entity type full name

[Ali-YousefiTelori]

• Fix the error detail when you try to add or update with an incorrect Entity type, In before we had the message of "Cannot create a DbSet for '{typeName}' because this type is not included in the model for the context." now we have full details of the message that is like "Cannot create a DbSet for 'EFCoreCheck.MySpace.User' because this type is not included in the model for the context.
  Your types:
  EFCoreCheckSpace.User,
  MyNewNameSpace.TransactionInfo"

[dnfadmin]

All CLA requirements met.

[ajcvickers]

@Ali-YousefiTelori How does knowing the types that are in the model help people resolve this error?

[Ali-YousefiTelori]

@ajcvickers In the past week, We got a problem with our project that got a developer one day time to find the problem and fix it.
What was the problem?

We had two entities with the same name but different namespace:

1.TransactionInfo in SQL Relational example: SqlNamesapce.TransactionInfo
2.TransactionInfo in NoSql Mongo example : MogoNamespace.TransactionInfo

With a developer mistake who use SqlNamesapce.TransactionInfo in DBSets of SqlDbContext but he sent MogoNamespace.TransactionInfo to context.Set... to get DbSet and add data to the database.
So we get that error. We didn't know we used different wrong TransactionInfo because the error does not show us the namespace of TransactionInfo.

DbSet<TInfo> table = context.Set<TInfo>();
await table.AddAsync(info);
await context.SaveChangeAsync();

I think these codes I pushed, will help the developer to know the Namespace of entities when they are setting the wrong type of context.Set and they will find the bug faster.

[AndriySvyryd]

Move this to Microsoft.EntityFrameworkCore.Metadata.Internal.EntityTypeExtensions

[Ali-YousefiTelori]

Fixed and moved

[AndriySvyryd]

Thanks for your contribution.