Add option to configure Finch pool :pool_max_idle_time

[jonatanklosko]

In some cases, a long-running app may want to make requests to arbitrary URLs (e.g. specified by the user). By default finch would start a separate pool for each host and keep it indefinitely, but this can be configured with the :pool_max_idle_time option.

Initially I wanted to just use a custom finch pool and do Req.new(finch: TransientFinchPool), however we also use :connect_options, and these cannot be used together. Hence, I'm proposing to add the Finch pool option. Thoughts?

[wojtekmach]

Thank you!

[wojtekmach]

As an aside, we probably should allow passing any Finch option (and let Finch fail if it's mistyped), I'll think about it some more.

[jonatanklosko]

I was thinking that maybe it should be nested pool_options: [pool_max_idle_time: 60_000] and allow any values, but since there are already different options converted to pool options, I am not sure.