Fix service bus queue SAS token generation

[jltorresm]

The PR to standardize HMAC implementation (#1473) introduced an issue in generating service bus queue SAS tokens.

The standard implementation in azure_core::auth::hmac assumes the signing key is the base64::decoded of the key passed as an argument:

azure-sdk-for-rust/sdk/core/src/hmac.rs

Line 12 in a32e411

let key = base64::decode(key)?;

On the service_bus/queue_client side it is used like the following, which means it'll be decoded inside the hmac function.

azure-sdk-for-rust/sdk/messaging_servicebus/src/service_bus/mod.rs

Line 67 in a32e411

let sig = hmac_sha256(&str_to_sign, signing_key)?;

However, from the version before the merge, I gathered the key needs to be used verbatim as bytes:

azure-sdk-for-rust/sdk/messaging_servicebus/src/service_bus/mod.rs

Line 67 in 346330b

let sig = hmac::sign(signing_key, str_to_sign.as_bytes()); // shared access key

Tested this using this example and confirmed that the fix works. If this is not the direction / style of fix you want, I'd be happy to rework it to make it more suitable.

I also wanted to add a few unit tests for the SAS token generation but this would need a time-mocking library so I wanted to check before changing the code further.

[demoray]

Thanks for the fix.