Change Auth struct signature field to Vec<u8>

[mnordstr]

The signature received from the nkeys::KeyPair sign function is a Vec<u8> and the contents cannot be stored in a String.
This commit changes the type of the signature field in the Auth struct from a Option<String> to a Option<Vec<u8>>.
This allows for JWTs to be used and signed with the nonce in the custom auth callback.

[Jarema]

Thanks for the PR!

I need to refresh my memory why it was String in the first place, because I have vague memory of having a reason for that :).

Nonethtless, could you please add a test for this?

[mnordstr]

Nonethtless, could you please add a test for this?

Test added.

[Jarema]

This looks good!

Please fix the format error and we're good to merge.

error[internal]: left behind trailing whitespace
   --> /home/runner/work/nats.rs/nats.rs/async-nats/tests/client_tests.rs:900:900:1
    |
900 |             
    | ^^^^^^^^^^^^
    |

warning: rustfmt has failed to format. See previous 1 errors.```

[mnordstr]

Please fix the format error and we're good to merge.

My bad, now cargo fmt is happy.

[Jarema]

LGMT! Thanks for your contribution!