Support multiple levels of config validation

Ideally, we wish to have levels of validation such as:
* strict
* basic (default)

In this case, strict validation will imply all the stringent checks are done (eg: #6644, #6300 ). basic might be closer to what we're doing today to allow for backwards compatibility of table and schema configs.

CC @mcvsubbu @mayankshriv

Start by reviewing the current validation behavior for table and schema configs, then read the related discussions in #6644 and #6300. Define how strict and basic levels are selected, which checks belong to each, and how the default preserves backwards compatibility; done means the behavior and scope are agreed.

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