Allow a single client instance to authenticate with multiple tokens

**What component of `google-cloud-cpp` is this feature request for?** For
example, is this related to bigtable (i.e., something in
`google/cloud/bigtable`), or GCS (i.e., something in `google/cloud/storage`)?

google/cloud/storage/client

GCS c++ client

**Is your feature request related to a problem?

Currently we need to create a new client per token. For our service we have multiple users with different tokens and having different clients per user is inefficient.
For GCS scale we should have a implementation where a single client can handle multiple auth calls (in ReadObj etc)

**Describe alternatives you've considered** A clear and concise description of
any alternative solutions or features you've considered.
NA

**Additional context** Add any other context about the feature request here.
NA

Start with the google/cloud/storage/client component and its ReadObj entry point. Determine how authentication is currently attached to a client and what behavior is required for multiple user tokens. Done means one client can handle authenticated calls for multiple tokens without requiring a separate client per token.

Written by the indexing model from the issue text.