restore: limit number of files restore span entries

[dt]

Today we size restore span entries solely based on the sum size of the files grouped into that entry, to hit the target size (384mb by default).

However, if a backup contains many small files, this can mean that a single span, to hit its target size, includes many files. Since all files in a span are allowed to overlap, we are obligated to open all of them to merge them. This, however, combines poorly with the above heuristic that can put a very large number (hundreds or thousands) of files into one span, as the restore processor then needs to open thousands of files concurrently to process that span.

We should have an additional secondary target limit on the number of files (64? 128?) in a span when constructing these spans.

Jira issue: CRDB-36313

[blathers-crl]

cc @cockroachdb/disaster-recovery