GCP components need a common auth layer

Currently, GCP components have inconsistent metadata properties, especially for authentication.

For example, this is the [GCP Storage Bucket binding](https://docs.dapr.io/reference/components-reference/supported-bindings/gcpbucket/) (alpha). Note the use of snake_case for the auth properties (which is anti-pattern in Dapr) - non-auth properties use camelCase:


This instead is the GCP Pub/Sub component. Note that all auth properties use camelCase (correctly), and that some properties like "type" are marked as deprecated:


As per @robertojrojas , using `privateKeyId` is an "old" method of authenticating with GCP and users should use "Application Default Credentials" instead.

In order to support all auth methods, and support various names for the metadata properties, we need a "common auth layer" for GCP components, like we have for Azure components (which is used to support all Azure AD auth methods and accepts "legacy" metadata names)