Support Token-based auth for GCP

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

Allow for authentication to through an access token for the [GoogleCloudStorageBuilder](https://docs.rs/object_store/latest/object_store/gcp/struct.GoogleCloudStorageBuilder.html).

**Describe the solution you'd like**

Possibly by adding support for access tokens to [GoogleConfigKey](https://docs.rs/object_store/latest/object_store/gcp/enum.GoogleConfigKey.html).

**Describe alternatives you've considered**

Our current work around involves creating a GcpCredentialProvider and then using the builder's [with_credentials](https://docs.rs/object_store/latest/object_store/gcp/struct.GoogleCloudStorageBuilder.html#method.with_credentials) to authenticate with an access token.

**Additional context**

Start with GoogleCloudStorageBuilder and GoogleConfigKey in the linked object_store GCP documentation, then compare them with GcpCredentialProvider and the with_credentials workaround described in the issue. Done means an access token can be supplied through the builder's normal configuration path and authenticates successfully without manually constructing a credential provider.

Written by the indexing model from the issue text.