Completions caching

The oracle databases I currently work with unfortunately have a ton of namespaces and tables.
This results in "Refreshing completions" for around a minute every time I start okcli.

I've talked a bit with @icepuma about this issue and we had the idea to add optional completion caching to okcli.
It could work like this:

• entirely opt-in and set by command line flag (eG --enable-completions-cache or similar)
• if enabled, the completions are serialized into the XDG_CACHE_DIR
• if enabled and there is a completions dump at start, deserialize from file instead of asking the DB
• if disabled, nothing changes

The cache would be per-connection, for example after hashing the connection string.
I don't have any particular preference regarding cache invalidation, maybe simply removing the file manually would be good enough for a first start, or a --force-invalidate-cache option (slightly better especially if the file name is a hash).

A slightly easier variation to avoid dealing with XDG would be to simply support --completion-cache <path> so the user can decide if and when to delete the file. But it also introduces the potential mistake of using a completion cache for a different connection which would be confusing.

What are your thoughts on this and would you accept a pull request implementing this?

Start at okcli startup and trace the completion-refresh path, then review how command-line flags and XDG_CACHE_DIR are handled. Done means an opt-in, per-connection completion cache can load saved data instead of querying the database, while disabled behavior remains unchanged.