Contacts Batch API Import

[yanchuk]

We need to support Contacts Batch API Import
https://api-docs.mailtrap.io/docs/mailtrap-api-docs/b46b1fe35bdf1-import-contacts

Import contacts in bulk with support for custom fields and list management. Existing contacts with matching email addresses will be updated automatically. You can import up to 50,000 contacts per request. The import process runs asynchronously - use the returned import ID to check the status and results.

[yanchuk]

@narekhovhannisyan it's unblocked