Feature Request

As defined by MDN (src):

The User-Agent request header is a characteristic string that lets servers and network peers identify the application, operating system, vendor, and/or version of the requesting user agent.

I'd like to request that sccache set a user-agent header for requests to AWS/S3. The user-agent header should include the sccache version.

Use Case

AWS supports writing IAM policies that block access to resources based on the user-agent header: https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_condition-keys.html#condition-keys-useragent.

I'd like to write a policy that denies access to an S3 bucket for versions of sccache that are affected by #2038.