whitelisted senders have unlimited allowance and proper checks for ft_transfer to burn address

[anthony-near]

• whitelisted senders have unlimited allowance on startup for fastauth. Fixes: https://github.com/near/pagoda-relayer-rs-fastauth/issues/17 by setting the initial allowance for all whitelisted senders to u64::max.
• proper check for ft_transfer to burn address for pay with FT
• rustfmt

[volovyks]

It's hard to predict if it works without integration tests TBH.
The fact that we are mixing concepts of senders and receivers is questionable.

[volovyks]

@anthony-near once the docker image is updated, we can test this functionality by rerunning tests here: near/mpc#347
For all of them to pass we will need to deploy it to dev env.

[anthony-near]

It's hard to predict if it works without integration tests TBH.

I know it's not as good as an integration test, but I ran it locally and called the /get_allowance endpoint after server startup (the "unlimited" allowance is initialized before the app routes are created) and it worked as expected.