feat(auth): rust client support for bearer auth

[matt-codecov]

mint tokens in our client library if a secret key and other config has been passed in

i will update #231 for end-to-end testing of both clients

depends on #236

Ref FS-202

[linear]

FS-202 Implement (optional) JWT in Clients

[jan-auer]

LGTM once rebased, thanks!

[lcian]

LGTM. Let's not forget to add this token_generator to the README as well.

[jan-auer]

We need to pass either an enum or a trait that has two implementations:

1. The TokenGenerator, as implemented.
2. A fully signed Token (could be a newtype around a string). This will be needed in sentry_cli.

There's a workaround currently that we configure the reqwest client builder and add a static header, but it would be much better that we make this a first-party functionality.

I'm good if this is added in a follow-up. Example for the trait in case you choose to implement it right now:

// disadvantage: Whatever we pass in must be public now. Maybe better to have a custom, internally borrowing struct.
pub use crate::client::ScopeInner as TokenScope;

pub trait TokenProvider {
  fn sign_for_scope(&self, scope: &TokenScope) -> crate::Result<Token>;
}

struct ClientBuilderInner {
  ...
  token_provider: Box<dyn TokenProvider>,
}

impl ClientBuilder {
  pub fn token_provider<T: TokenProvider>(self, provider: T) -> Self {
    let Ok(mut inner) = self.0 else { return self };
    inner.token_provider = Some(Box::new(provider));
    Self(Ok(inner))
  }
}

[matt-codecov]

technically i left it out of the README but it is added to the example client construction in the doc comments.