pageserver: handle aux file keys in imports

[skyzh]

currently it does not work with the page server import functionality https://neondb.slack.com/archives/C033RQ5SPDH/p1719553893686989?thread_ts=1719403793.586119&cid=C033RQ5SPDH

We need some logic to auto-detect if there exists v2 aux file keys during the import. If yes, we should put last_aux_file_policy = v2 in the index_part file; or forcing v2 for all imported tenants.

[jcsp]

or forcing v2 for all imported tenants.

That sounds fine to me. The import API isn't used in prod under normal circumstances, and if it doesn't work for people with weird beta-feature stuff in their databases that's fine.

[knizhnik]

Please notice that right now import_file just ignores all AUX files (replication slots,...).
So if we are importing data from tar, then this files are missed.
It was fixed in #6560

[jcsp]

We can also consider removing this API, as it is not used in production. Last time we discussed removing it there was concern that it was used in some support cases by the compute team.