Verify type assumptions when retrieving child default values

[liamcervante]

This PR fixes the root cause of hashicorp/terraform#32752.

Essentially, when the user passes in a type that cannot be converted into one assumed by the defaults package we would see a panic when attempting to retrieve the children of the mismatch type. For example, if we are attempting to convert a list into an object (which is not possible and what is happening in the original issue) then the defaults package would panic assuming an int type should be a string as that is what the index was expecting.

This PR makes that assumption safe by verifying both the expected type of the key and the cty type expected by the defaults package match. If they don't match, the defaults package will simply not apply the defaults (by returning nil from the getChild function, as if there were no defaults to apply). As we expressly tell callers they should attempt to convert from the returned value into the concrete required type, this error would be caught then and handled nicely instead of the panic we see now.