Enhancing Custom Type Registration in pgx

I've created PostgreSQL extension that adds native unsigned integer data types to PostgreSQL.
I also implemented support of these types in pgx.

However implementing pgx part was a bit painful because I have need to copy & paste pgx internals code:

https://github.com/jackc/pgx/blob/2ec900454bfe65daa9648488e93f7627c26b810c/derived_types.go#L249-L262
https://github.com/pg-uint/pgx-pg-uint128/blob/d194dc5568275d1aabc0a6900f4311012e21abdf/types/register.go#L241-L257

https://github.com/jackc/pgx/blob/2ec900454bfe65daa9648488e93f7627c26b810c/pgtype/register_default_pg_types.go#L5-L35
https://github.com/pg-uint/pgx-pg-uint128/blob/d194dc5568275d1aabc0a6900f4311012e21abdf/types/register_default_pg_types.go#L31-L62

And there's also lack of ability to register base types:
https://github.com/jackc/pgx/blob/2ec900454bfe65daa9648488e93f7627c26b810c/derived_types.go#L158-L163

So, I have need need to implement types registration on my own:
https://github.com/pg-uint/pgx-pg-uint128/blob/d194dc5568275d1aabc0a6900f4311012e21abdf/types/register.go#L66-L122

https://github.com/pg-uint/pgx-pg-uint128/blob/d194dc5568275d1aabc0a6900f4311012e21abdf/types/register.go#L124-L130

To streamline the process, I suggest we export some of these functions, such as registerDefaultPgTypeVariants and serverVersion (perhaps as a member method of conn).

Additionally, we could either adapt the LoadTypes function to support loading base types or provide a new API for base type registration.

Start with derived_types.go and pgtype/register_default_pg_types.go, especially the referenced registration functions and LoadTypes behavior. Compare those paths with the corresponding implementations in pgx-pg-uint128 to understand the duplicated internals and base-type limitation. Done means an agreed public registration API supports custom and base types without consumers copying pgx internals.