Exchange your Payrails access token for a Vault access token.

The Vault access token authenticates the tokenize and detokenize endpoints served on the Payrails
Vault host, and grants only the operations your Payrails access token permits. The request body is
empty, and these tokens are short-lived (the exact lifetime is returned in expires_in). Store it
until it expires, then request a new one.

This endpoint is available by request and requires an approval by Payrails.