CustomFieldStattable

Clients of @PublicSpi can expect that programs compiled against a given version will remain binary compatible with later versions of the @PublicSpi as per each product's API policy (clients should refer to each product's API policy for the exact guarantee -- usually binary compatibility is guaranteed at least across minor versions).

Note: @PublicSpi interfaces and classes are specifically designed to be implemented/extended by clients. Hence, the guarantee of binary compatibility is different to that of @PublicApi elements (if an element is both @PublicApi and @PublicSpi, both guarantees apply).