Update voucher client to use tokens for authentication

[lynnsh]

Client:

• added idtoken library automatically adds the authorization token to http client given the audience url (voucherURL)
• idtoken library only works with service accounts (as per https://github.com/googleapis/google-api-go-client/issues/873), so the client reverts to http client if this is not the case

Server:

• do not use basic auth if run in CloudRun

[rxbchen]

Let's remove the reference to the internal issue 😄! Since this is a public repo

[thepwagner]

LGTM, just an idea to reduce.