Add support for more cache TTL control for per-package tuning

[jamietanna]

Tell us more.

For instance, if a self-hosted administrator wishes to get an update to ghcr.io/renovatebot/renovate, without expiring their full cache or hacking around in Redis, there's no way, aside from waiting until the soft TTL is expired.

In this case, having a per-package override would be useful - if possible - as it would provide more control for more sensitive packages.

Alternatively, a self-hosted config option to say "trust me, ignore the cache and go check this package in the registry"