Support for using config variables in yaml config

[FrittenKeeZ]

Description

Shameless copy/paste from statamic-rad-pack/mailchimp#137
PR: statamic/cms#11403

Before migrating to v5 we could use dynamic audience_id based on our environment using the env:

'audience_id' => env('MAILCHIMP_AUDIENCE_ID'),

After upgrading I wanted to accomplish the same using the config, but it breaks everything because the config value isn't being replaced

audience_id: '{{ config:mailchimp:audience_id }}'

Taken from API call:

"lists/{{ config:mailchimp:audience_id }}"

Steps to reproduce

1. Add audience_id to mailcihmp.php config
   'audience_id' => env('MAILCHIMP_AUDIENCE_ID'),
2. Change audience_id in newsletter.yaml to use config
   audience_id: '{{ config:mailchimp:audience_id }}'
3. Navigate to /cp/forms/newsletter/edit
4. See that you get a fatal error
   StatamicRadPack\Mailchimp\Fieldtypes\MailchimpField::callApi(): Return value must be of type ?array, bool returned