[4.x] OAuth improvements including support for SAML2 providers

[duncanmcclean]

This pull request fixes two issues preventing our Socialite/oAuth integration from working with SAML2 Socialite Providers, like Auth0/Okta.

Essentially, there were only two changes required to make our code work with SAML2:

• The /oauth/{provider}/callback route needs to accept POST requests, not just GET requests.
• When we're fetching the user via Socialite, we needed to call ->stateless(), otherwise we'd get an error.
  • However, the ->stateless() option only works for some providers and not all of them. There's also no way for us to know programatically if a specific provider supports it or not - you need to simply test it and see.
  • To workaround this, I've made it possible to specify stateless as a config option in the providers array in config/statamic/oauth.php... more details on that below

Provider "Configs"

You can configure which providers Statamic should use for oAuth in the providers array in config/statamic/oauth.php. You can optionally configure labels for providers too.

'providers' => [
    'github',
    'saml2' => 'Okta',
],

Like I mentioned, only some providers support the ->stateless() method, so I've made it so you can pass an array of config options:

'providers' => [
    'github',
    'saml2' => ['stateless' => true, 'label' => 'Okta'],
],

Most of the code changes in this PR are around making the config stuff work. Happy to re-work if you think there's a better way to do this.

---

Fixes #9535.

[jasonvarga]

I made some more tweaks:

• Provider::label() and Provider::config() are now just getters.
• Provider::label() is still a setter for backwards compatibility, but the setter/property is marked as deprecated. The label should now just be passed through the config.
• Added tests for those since they were missing.
• Use OAuth::provider() in UserRepository to ensure new Provider only happens in one place: Manager.php. Otherwise, that instance would be missing config which might be important.