Signed URL support

[grindarius]

Hello and thank you for this amazing library.

I've been working with this library for quite a while and it's really good. I wanna ask is it possible to support signed url generation for google cloud storage with this crate? I am willing to contribute to make it happen. Thank you.

[abdolence]

Hey,

Seems you have everything needed available:

1. You need to creating an URL to sign (kind of rewrite this example for Python): https://cloud.google.com/storage/docs/access-control/signing-urls-manually#python-sample)

2. To sign you can use IAM client (or you can actually use any library that supports RSA signatures with SHA-256):

   let client: GoogleApi<IamCredentialsClient<GoogleAuthMiddleware>> = GoogleApi::from_function(
       IamCredentialsClient::new,
       "https://iamcredentials.googleapis.com",
       None,
   )
   .await?;
   
   let resp = client
       .get()
       .sign_blob(
           tonic::Request::new(
               gcloud_sdk::google::iam::credentials::v1::SignBlobRequest {
                   name: "projects/-/serviceAccounts/<your-sa@email>".to_string(),
                   delegates: vec![],
                   payload: canonical_req.as_bytes().to_vec(),
               }
           )
       )
       .await?;

If you want to make an auxiliary function to help others, feel free to open PR for sure :)

[grindarius]

or you can actually use any library that supports RSA signatures with SHA-256

Does this mean I need to extend the Credentials enum, and if the service accout is present, load it into ring and use ring to sign right?

https://github.com/abdolence/gcloud-sdk-rs/blob/db03cc94e48ac7b1b02aab54711e3b4fbb4fb2ca/gcloud-sdk/src/token_source/credentials.rs#L14-L19

[abdolence]

If you can use signBlob it is preferable (more secure setup), so you don't have to introduce and manually rotate JSON private keys.

Otherwise, no need to introduce any other type, you just need to read JSON file using existing functions like from_well_known_file/from_file/fron_json to read Credentials.