Add an API endpoint for auto-translating custom texts.

Current state:

Currently, there is no option to use auto-translation characters outside the project.

Goal:

The goal is to provide a simple-to-use endpoint (e.g., POST /v1/auto-translate/) to auto-translate custom texts sent from the client. It would be good to have a caching option, so the same texts would not decrease auto-translation characters from the account balance. The endpoint would be mainly used for apps with small or medium traffic and would be rate-limited based on IP address. It would require Project API Key authentication.

Reason:

The main reason for such an endpoint is to give users the simple option to auto-translate custom texts, e.g.: user comments and other user-generated content.