backupccl: fix bug in resolving encrypted backup manifests

[adityamaru]

In #87311 we refactored ResolveBackupManifests to concurrently load manifests from base and incremental layers by calling FetchPreviousBackups. The mistake in that diff was to pass in only the incremental layers of the backup to FetchPreviousBackup instead of passing in the base backup + incremental layers. This was silently passing all tests because in ResolveBackupManifests we explicitly setup the base backup layer in all the relevant slices before processing each incremental layer. The one case this was not okay in was encrypted backups. FetchPreviousBackups apart from doing the obvious would also read the encryption options from the base backups before reading the manifests from each layer. Now, because we stopped sending the base backup in the input slice, this step would fail since the method would go to the first incremental backup (instead of the base backup) and attempt to read the ENCRYPTION_INFO file. This file is only ever written to the base backup and so a SHOW BACKUP or a RESTORE of an encrypted backup would fail with a file not found error.

In this diff we:

1. Fix ResolveBackupManifests by passing in the base backup + incremental backups to FetchPreviousBackups.

2. Make it the callers responsibility to pass in the fully hydrated encryption options before calling FetchPreviousBackups so that the method is only fetching backup manifests.

Fixes: #91886

Release note (bug fix): fixes a bug that would cause SHOW BACKUP and RESTORE of encrypted incremental backups to fail

[cockroach-teamcity]

This change is 

[adityamaru]

I'm going to add some tests that should've caught this (mis)behaviour when the original PR was merged.

[stevendanna]

This looks reasonable to me. We really need some tests here.

[stevendanna]

Thanks for adding the test!

[adityamaru]

blathers backport release-22.2

[adityamaru]

blathers backport release-22.2.0

[adityamaru]

TFTR!

bors r=stevendanna

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

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