fix: improve association decorators

[ephys]

Pull Request Checklist

• Have you added new tests to prevent regressions?
• If a documentation update is necessary, have you opened a PR to the documentation repository?
• Did you update the typescript typings accordingly (if applicable)?
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Description Of Change

This PR:

• Forbids specifying the as option at the root of association decorators, because it will be ignored in favor of using the name of the property being decorated
• Loosens the typing of the @BelongsTo decorator, due to it being incompatible with inheritable models