Add CacheBehavior::AUTOMATIC to DBInstanceCache that automatically does the right thing™ - #19841

This PR adds a CacheBehavior enum to the instance cache, instead of the current true / false:

enum class CacheBehavior { AUTOMATIC, ALWAYS_CACHE, NEVER_CACHE };

The default value if none is supplied is CacheBehavior::AUTOMATIC. This does what you would expect:

• Unnamed in-memory connections are not cached (:memory: and <empty>)
• Named in-memory connections are cached (:memory:label)
• Regular connections are cached

This PR also switches the C API to using CacheBehavior::AUTOMATIC.