Fetch custom type oids - use case: pgvector extension

In order to use vector type of the pgvector extension, we currently need to start a temporary connection to the db to get the vector type oid and then add it in the options types. See https://github.com/porsager/postgres/issues/1041

Similar to fetchArrayTypes, it should be possible to get the oid to fill to and from properties of the custom type and thus make it look like this

let options = {
  types: {
    vector: {
      serialize: v => JSON.stringify(v),
      parse: v =>JSON.parse(v)
     }
  }
}

This OID lookup would only happen for types that don't have to and from defined. Or maybe only under a flag in the type definition: fetch_oid: true

Start by reading the existing fetchArrayTypes behavior and the custom types options described in the issue. Determine how missing to and from properties should trigger an OID lookup, then verify the result with the pgvector vector type example; done means custom types can serialize and parse without a temporary connection.