Providing a non-StacApiIO instance when creating a client will cause many methods to throw exceptions

[philvarner]

If a Client is created with the from_file static method and a StacIO instance that is not a StacApiIO instance is provided as an argument, many methods will raise method not found errors.

The reason is that many methods call conforms_to method on the StacIO instance, which is only a method in StacApiIO and not
pystac.StacIO.