feat(pubsub): add list configs for topic & sub

[hongalex]

When calling client.Topics and client.Subscriptions, you get back an iterator that allows you to iterate over all resources. However, this gives you the actual Topic and Subscription struct that's defined by the library and does not list configs, which currently need to be retrieved from an additional config call. This adds NextConfig to both Topic/SubscriptionIterator so that the config can be viewed instead of Topic/Subscription. This is the desired behavior, and is used for SchemaConfig as well as topic/subscription configs in Pub/Sub Lite.

Thanks to @mgabeler-lee-6rs for suggesting this.

Closes #4559

[noahdietz]

LGTM but another pubsub reviewer might want to approve

[mgabeler-lee-6rs]

@hongalex I just tried to pick this up in my app, and ... I realized I missed something pretty important in my original suggestion: the SubscriptionConfig and TopicConfig objects returned from these don't have any information about the identity of the returned objects, which makes them nigh useless 🤦‍♂️

What do you think about adding a name field and String() and ID() methods to TopicConfig and SubscriptionConfig to match Topic and Subscription?

[hongalex]

Oof, yeah you're totally right. I think the original intent for Topic/SubscriptionConfig was purely for storing config (regardless of the actual topic/config it is associated with). I'll draft up a fix for this today. Thanks for reporting as always!

[mgabeler-lee-6rs]

@hongalex guessing this got lost in the shuffle (no hard feelings!). Want me to put an issue up for the followup fix?

[hongalex]

@mgabeler-lee-6rs apologies, indeed I created a starting point but slipped through the radar. I just created #4952 to track this better.