Validate the blueprint schema when we try to activate a blueprint sent from SDK

[jleibs]

What

• Partially addresses: Be extra thorough testing version switching between 0.15 / 0.16 and trying to load cross-version blueprints #6213

We already used to validate on load from file, but this pulls the validation into the default/activation pathways.
This keeps us from getting into a state where we can't write to the blueprint due to components with inconsistent schemas.

• Future work: Report information about version mismatch when blueprint schema fails to validate #6282

Unfortunately we can still get issues when opening 0.16 blueprints in 0.15 viewers since 0.15 is missing this validation.

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