Support configuration via env

[skryukov]

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

There are a couple of things I wanted to address regarding #172:

• Usually ENV values take precedence over values set in the initializer
• 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 → InertiaRails.configure → ENV → controller-level config

Example:

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

# env
ENV['INERTIA_SSR_ENABLED'] = 'true'
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.