Feature request: improved dynamic_command support

[jfly]

Issues

• I have checked existing issues and there are no existing ones with the same request.

Feature description

While working on #192, I learned about none-ls's dynamic_command functionality, which I understand is often paired with the by_bufnr cache helper. This works great, but it does mean the very first time I save a buffer, I have to wait for my dynamic_command to generate the command. After that point, things are cached, and life is great. I think things could be even better, though:

I'd like have none-ls to start computing the dynamic_command the moment I open a buffer. Since I usually spend some time reading a file before making a change, I expect this to make the dynamic_command delay disappear in practice (by the time I make a change and try to format my buffer, we'll have already computed the dynamic_command.

Help

Yes

Implementation help

I'd be happy to implement this.

• Is this setup_buffer function the right place to trigger the dynamic_command lookup?
• Rather than adding a new option for this, I'd like to just enable this for all formatters with a dynamic_command that uses a cache helper. Right now, I don't think it's possible to check if a dynamic_command uses a cache helper, but I think we could achieve that by reworking our cache helper function to return a "callable table".
• I'd then rework the code that currently invokes dynamic_command to instead wait for any ongoing dynamic_commands to finish so we don't trigger a second one while the original one is going. Conveniently, it doesn't actually matter how many ongoing dynamic_commands there are, we just need to wait for them to finish, and then we can invoke dynamic_command knowing we'll get an instantaneous response.
• Bonus points: I think we could give the user a slick progress notifcation using LSP's Progress Support, which I learned about from https://github.com/j-hui/fidget.nvim. Basically, we can give a progress bar/spinner while we're invoking the dynamic_command for the current buffer.