Add message casters for Protobuf descriptors

[yuri-sergiichuk]

The (distinct) option applied on enum values fails to actually convert an enum value to Any because of the constraint iterates over EnumValueDescriptor instead of actual enum values.

In order to support consistent behavior across all possible types, we should consider adding appropriate message casters for such a situation.

[alexander-yevsyukov]

@yuri-sergiichuk, so we cannot put (distinct) for a repeated enum field right? If so, it's a bug.