Scanner - GCP SecretManager

Introduce code to permit the retrieval of sensitive configuration values from Google Cloud Secret Manager.
Code will be implemented in ConfigKeys class.

Preconditions for GCP Implementation - Workload Identity Federation:
- A Windows System Environmental variable needs to be created on the scanning workstation called "GOOGLE_APPLICATION_CREDENTIALS". The value of this variable is to provide the location of a credential JSON file which contains the Application Default Credentials (ADC) for use by Cloud Client Libraries.
- The Google.Cloud.SecretManager.V1 Nuget package needs to be installed on the AppConfiguration project.
- The Workload Authenticator for Windows needs to be installed on the scanning workstation. The authenticator is a tool used to allow Windows-based applications to authenticate to Google Cloud services by utilizing existing Active Directory (Kerberos) credentials.

LINKS:
[https://googlecloudplatform.github.io/iam-federation-tools/wwauth/](https://googlecloudplatform.github.io/iam-federation-tools/wwauth/)
[https://docs.cloud.google.com/docs/authentication/client-libraries](https://docs.cloud.google.com/docs/authentication/client-libraries)
[https://docs.cloud.google.com/iam/docs/workload-identity-federation](https://docs.cloud.google.com/iam/docs/workload-identity-federation)