Move out the metadata update checks in _utils.py

[R-Palazzo]

Problem Description

Both BaseSynthesizer and BaseMultiTableSynthesizer contain a method called _check_metadata_updated. This method is intended to raise a warning if the metadata has been updated before the synthesizer definition or before fitting it. We are planning to move this logic to sdv/utils/_utils.py so that it can be used widely.

Expected behavior

• Move out _check_metadata_updated from BaseSynthesizer to sdv/utils/_utils.py
• Rename it _check_single_table_metadata_updated
• Move out _check_metadata_updated from BaseMultiTableSynthesizer to sdv/utils/_utils.py
• Rename it _check_multi_table_metadata_updated
• Update the calls in the synthesizer __init__() and fit() to use the correct method.

Additional context

Update the tests and docstring accordingly.