Do not call orm.Model.Meta callables during `__init_subclass__`

[mesozoic]

The whole idea behind allowing Meta values to be callables (#268) was to defer evaluation until after the Model subclass is created. However, these callables still get evaluated when the subclass is created, since we try to validate that base_id, table_name, and api_key are defined.

This branch will validate that those three names exist inside Meta, but will defer raising an exception if they are callable and return None until the first time they get used.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (eef7b6f) to head (c289427).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #357 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 25 25 Lines 2313 2315 +2 ========================================= + Hits 2313 2315 +2 ```

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