Add extensions methods for MigrationBuilder to rename primary keys/foreign keys.

[GillesTourreau]

Currently Entity Framework migration does not support to rename primary keys/foreign keys (see dotnet/efcore#5662 issue).
Add and provides the following extensions methods for the MigrationBuilder :

• RenamePrimaryKey(name, newName, table, schema)
• RenameForeignKey(name, newName, table, schema)