CosmosDB: support for changing AuthorizationToken in pipeline

[MindFlavor]

CosmosDB crate allows to change the AuthorizationToken without having to recreate the client(s). This is useful for quickly switching security context.
The relevant code didn't account for the new pipeline architecture. This PR fixes that (so now even the permission_token_usage test case works as expected).

Fixes #337.

[rylev]

This seems pretty error prone... I wonder if there's a better way to do it... Perhaps something like this could be better:

  /// Set the auth token used
    pub fn auth_token(&mut self, auth_token: AuthorizationToken) {
        self.auth_token = auth_token.clone();

        let new_auth_policy: Arc<dyn azure_core::Policy<CosmosContext>> =
            Arc::new(crate::AuthorizationPolicy::new(auth_token));
        use std::any::Any;
        let auth_policy = self
            .pipeline_mut()
            .policies_mut()
            .iter_mut()
            .find(|p| (**p).type_id() == TypeId::of::<AuthorizationPolicy>())
            .unwrap(); // TODO: add auth policy if one doesn't exist?
        std::mem::replace(auth_policy, new_auth_policy);
    }

I've not tested whether this works, but I think it should.

[MindFlavor]

Yes it is error prone, unfortunately we forego the type safe approach so these problems are bound to happen.
In this instance, I think we should get away with the simpler code since:

1. The only place we construct the pipeline is new_pipeline_from_options.
2. In new_pipeline_from_options, we explicitly add the AuthorizationPolicy as last retry policy (penultimate position).

Since there is no way to construct a pipeline without an AuthorizationPolicy at the penultimate position my proposed code will (should? 🤣) always work.

On the other hand, your code is safer but it would correctly work on a buggy pipeline (AuthorizationPolicy must be the last retry policy for it to work properly) so I think it's a moot point.

What about an assert? Maybe a debug_assert that gets removed from production code?
The assert should enforce that the swapped policy is indeed an AuthorizationPolicy.