This patch adds Admin support for the listTransactions API, which was added by KIP-664. Similar to listConsumerGroups, the new listTransactions API is intended to be sent to all brokers. This did not fit very well with the new AdminApiDriver that was used for the other KIP-664 APIs, so I had to refactor a bit. The way I ultimately handled it is to treat the brokerId as the key that needs to be mapped. Unlike with the other cases, the set of brokerIds is not known ahead of time and has to be discovered through a Metadata lookup (just as with listConsumerGroups). Because the future associated with ListTransactionsResult is more complex, I ended up externalizing future completion in a new AdminApiFuture object which is passed to AdminApiDriver during construction.

One minor change that is also worth calling out is the removal of AdminApiHandler.Keys. This was previously used to indicate the sets of statically and dynamically mapped keys. Instead, I now handle this with a new StaticBrokerStrategy. The static scope is indicated by returning an ApiRequestScope object which has a specific destination broker set.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)