ext.NativeTypes: Recursively add sub-types

[alvaroaleman]

This change extends the NativeTypes provider to not only add the passed-in type but also all of its sub-types in order to simplify using it in the context of nested structs.

Fixes https://github.com/google/cel-go/issues/885

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[TristonianJones]

/gcbrun

[TristonianJones]

/gcbrun