feat(pubsublite): support seek subscription in AdminClient

[tmdiep]

SeekSubscription performs an out-of-band seek for a subscription to a specified target. Adds SeekSubscriptionOperation, which wraps the auto-generated gapic type for polling the seek operation.

[tmdiep]

@codyoss, @tbpg - please review this API change.

I couldn't find other examples of handwritten libraries exposing operation polling. We want to allow waiting for the operation to be optional, as users may want it to be asynchronous in most cases.

Notes:

• pubsublite.SeekSubscriptionOperation basically just wraps the gapic generated operation, transforming Timestamp protos to time.Time.
• SeekTarget implements the equivalent of a oneof.
• SeekSubscriptionOption is a placeholder for future options (which we've deferred). It looks a little strange to define now. But I wanted to avoid introducing another variant of SeekSubscription.

[codyoss]

A couple of comments, Looks good overall though!