We should release our policy types

[dbolduc]

We define policy types like this: https://github.com/googleapis/google-cloud-cpp/blob/8a2efcfa34e7b476654de534312d742041ded916/google/cloud/accessapproval/access_approval_connection.h#L37-L39

This is in violation of https://github.com/googleapis/google-cloud-cpp/blob/main/doc/adr/2022-01-21-only-public-types-in-public-APIs.md

There is no way to learn the interface of this type, other than by looking at the code. (Doxygen does not generate documentation for internal::TraitBasedRetryPolicy<T>)

To allow for customer-defined retry policies, I think we need to release RetryPolicy and TraitBasedRetryPolicy.