backupccl: download pre restore data in cluster restore

[msbutler]

This patch adds the pre restore data spans to the list of spans to download.
While these pre restore spans map to data in the temporary system table
database that are then rewwritten to the actual system table, the download job
ought to download all external data linked into the cluster out of principle.

Fixes #124330

Release note: none

[cockroach-teamcity]

This change is 

[dt]

These tables are already dropped and awaiting GC by the time we write the download job; why are we using download bandwidth and iops on something just to delete it?

[msbutler]

I don't really have strong opinions here, but i have 3 reasons:

• it is nice to guarantee that after the download job completes, no external bytes are left on the cluster
• It's also only a mbs of pre restore data, though perhaps it scales for larger clusters.
• It would also be nice for our check to pass on cluster restore:

`SELECT
COALESCE(stats->>'external_file_bytes','0') FROM
crdb_internal.tenant_span_stats( ARRAY(SELECT(crdb_internal.tenant_span()[1], crdb_internal.tenant_span()[2])))`

I'm also happy to skip the check on cluster restores and move on though.

[dt]

As long as the "pre" spans are after the main spans I guess it doesn't matter; in a non-trivial restore they're already GC'ed and the vSSTs are gone anyway so it is a noop.

[msbutler]

TFTR!

bors r=dt

[craig]

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