Validate output of functions against type

[w33ble]

Describe the feature:

Function definitions specify a type, which informs the interpreter about what type of data the function will produce. However, the output when calling a function is never validated against that type.

Canvas has type validation, which can be used in addition to a basic getType check, if we want to get fancy. But a simple getType(output) === fnSpec.type would suffice.

Describe a specific use case for the feature:

This is mostly so that when users create custom functions, they can be sure that they thing they return is valid, and will work with other functions that handle that type.

This is going to be an issue with some existing functions in Canvas though, specifically the filter function, as noted in #37422

[elasticmachine]

Pinging @elastic/kibana-canvas

[elasticmachine]

Pinging @elastic/kibana-app-arch

[lukeelmers]

Hopefully the TS ExpressionFunction generic from #37438 helps with this too, as it will enforce the shape of the return value (of course, that's relying on the user to import and specify their types correctly, so certainly not bulletproof) :)

[ppisljar]

Thank you for contributing to this issue, however, we are closing this issue due to inactivity as part of a backlog grooming effort. If you believe this feature/bug should still be considered, please reopen with a comment.