Use `Authorization` subject in gRPC and HTTP authz

### Context

Following https://github.com/influxdata/influxdb/pull/26429, upstream changes to the `Authorizer` trait and related types were brought in that introduce the ability to associate a "subject" with a set of permissions. This is accomplished by the [`Authorization`](https://github.com/influxdata/influxdb3_core/blob/fd0e474a6c0af5ba867399d753f5df18f59907cb/authz/src/authorization.rs#L3-L10) type returned by the [`Authorizer::authorize`](https://github.com/influxdata/influxdb3_core/blob/fd0e474a6c0af5ba867399d753f5df18f59907cb/authz/src/authorizer.rs#L13-L37) method.

The subject was left as `None` in https://github.com/influxdata/influxdb/pull/26429.

### Purpose of this issue

Determine the correct use of the `subject` and make consistent use of it in gRPC and HTTP authz flows.

### Suggestions

* Stringified `TokenId` of the token associated with the permissions
* The name of the token associated with the permissions

Start by reading the Authorization type and Authorizer::authorize method referenced in the issue, then trace how authorization is handled in the gRPC and HTTP flows. Determine the intended subject value, document the decision, and make its use consistent across both flows.

Written by the indexing model from the issue text.