Validate output data matching expected schema from sources / functions

[georgeh0]

Currently, for sources / functions implemented in Rust, since the executor code directly construct values in the generic cocoindex::value::Value type, there's no enforcement to make sure the value type actually matches the schema declared by the executor.

Downstream logic just assumes value's type matching the expected schema. If not true, there can be hard to debug problems. #725 is a recent example.

We want to:

• For each source / function, validate its returned value matched the declared schema. It should happen within the evaluator. Raise an error with clear message if mismatch.
• Only do this validation under "cocoindex debug mode". We can let users to set a COCOINDEX_DEBUG environment variable for users to turn on this debug mode.

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