Add minimum pooled parsercache sections check
With this change, the dbctl generated-dbconfig validity check will fail if fewer than parsercache_min_pooled_sections (default: 2) parsercache sections are pooled in a given datacenter.
The minimum pooled sections threshold can be set via the conftool config extensions_config section's dbctlconfig (also new). This also provides a mechanism to disable the check (setting parsercache_min_pooled_sections to 0 therein).
Note that, like most other dbconfig validity checks, this is not wired into the restore path, as the latter must operate without the benefit of trustworthy section objects (i.e., it only asserts very basic structural validity).
Bug: T383324