Migrate to a different method of storing error data

[johnwatkins0]

Feature description

This is closely related to #5552. Leaving this description intentionally vague for now because I believe others will have better ideas than me about the implementation details.

Currently, validation error data is stored in the amp_validated_url post type and amp_validation_error taxonomy in a way that makes the data hard to query, with much of the data JSON-stringified and stored in post content and term descriptions. We should implement a new way of storing the data that doesn't involve using JSON strings this way and makes it easier to perform more complex queries against the data.

In a recent discussion, custom tables were mentioned as one option.

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

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