Fallback provider

Problem/Motivation

I use TMGMT module in headless scenario, since JSON:API has limitation when it comes to translation creation ( https://www.drupal.org/docs/core-modules-and-themes/core-modules/jsonapi... → ) i find this module very important in the whole setup. I use DeepL as Provider, free plan which has limitations to 500k characters. In case when i reach character limit for the plan, translations are not created, since JSON:API has limited POST support i need something to create translation even with source language. I am aware of Drupal language fallback mechanism but this does not apply to headless scenario where i need to edit an entity after creation on other than source language. So the requirement is to add possibility to create a translation, even with source contents, in case defined providers failed to translate.

Steps to reproduce

Proposed resolution

Would be great to add possibility to define on each Provider a "Fallback provider", so in case when defined provider translation plugin failed to translate, then use the fallback one. Or maybe this can also be done the other way on providers list where we can set priorities to define global "Fallback provider", with lowest priority, to be used in case any translation Exception was thrown for higher priority Providers.

Remaining tasks

User interface changes

API changes

Data model changes