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

Comienza con el componente google/cloud/storage/client y su punto de entrada ReadObj. Determina cómo se adjunta actualmente la autenticación a un cliente y qué comportamiento se requiere para varios tokens de usuario. Se considera terminado cuando un cliente puede gestionar llamadas autenticadas para varios tokens sin requerir un cliente separado por token.

Escrito por el modelo de indexación a partir del texto del issue.