cloud_storage: API to disable and purge tiered storage

[jcsp]

Setting redpanda.remote.write=false stops the upload process, but does not clear anything up.

If a topic is having issues with tiered storage, or simply was enabled mistakenly, we should provide a way to disabled tiered storage that entirely cleans up & leaves the topic in the same state it would have been if tiered storage had never been used to begin with:

• Remove all objects in S3.
• Erase contents of archival_metadata_stm in the raft group.
• Also toggle the tiered storage properties to off on the topic (this could be done separately, but doing it all in one admin API call will much reduce the chance of someone mistakenly purging a topic and then leaving it in a state where it would immediately start writing to S3 again).

JIRA Link: CORE-1122

[mattschumpert]

@jcsp we have now seen multiple beta testing users desire this, in the course of toggling on and off TS for existing topics (testing) and in the course of migrating from one object store type/location to another. There are workaround but have been messier and time consuming. So +1 to this