Add a warning if you're loading a SingleTableMetadata object

[npatki]

Problem Description

In an upcoming version of SDV, we will be consolidating the Multi and Single table metadata objects into a streamlined Metadata object. Effectively, this object is the same as multi-table. So for single-table, it will start including a table name.

However for backwards compatibility reasons, a user should still be able to load in the SingleTableMetadata object into the new Metadata format. When doing so, we assign a placeholder table name since one was not available in the older object. Currently, this is not transparent to the user -- they do not know this is happening and what the placeholder table name is.

Expected behavior

Throw a warning whenever a SingleTableMetadata object is loaded into the streamlined Metadata format. Explain that a placeholder table name will be supplied, and what the name is.

from sdv.metadata import Metadata

metadata = Metadata.load_from_json(
    filepath='my_single_table_metadata.json'
)

Warning: You are loading an older SingleTableMetadata object. This will be converted
into the new Metadata object with a placeholder table name ('table').
Please save this new object for future usage.