Support configuration via env

[skryukov]

This PR generalizes and improves upon the implementation from #172.

I would love to see a more general approach to control configuration values (e.g., ssr_enabled)

Having the first point in mind, I needed to choose a strategy to prioritize gem defaults, global user configuration, env options, and finally controller-level configuration.

This is what I came up with, trying to follow the principle of least surprise:
InertiaRails.defaults → ENV → InertiaRails.configure → controller-level config

Example:

# initializer
InertiaRails.configure do |c|
  c.ssr_enabled = true
  c.ssr_url = 'localhost:1234'
end

# env
ENV['INERTIA_SSR_ENABLED'] = 'false'
ENV['INERTIA_SSR_URL'] = 'vite:1234'

# controller-level
class PublicController < ApplicationController
end

class DashboardController < ApplicationController
  inertia_config(ssr_enabled: false)
end

With this configuration, SSR will be enabled for PublicController but disabled for DashboardController.

[brodienguyen]

I am sold with this approach. Love your work as always bro @skryukov
Happy to close my PR to focus on pushing this approach forward.

[BrandonShar]

Usually ENV values take precedence over values set in the initializer

Is this the most common pattern? I would've guessed that manually set options in an initializer should override the environment because they're explicitly set. Everything else makes perfect sense to me!

[skryukov]

I think you're right @BrandonShar, that will be more Rails-y, will update the PR.

[bknoles]

Just to confirm, this one is ready to go yea? Last commit looks to resolve the conversation nicely!

Would be nice to get this into the docs after we get it released!

[skryukov]

Yup, it's ready 👍