Test `Credentials JSON` authentication method

[orouz]

Motivation

when users install CSPM GCP and select manual deployment, they need to provide a service account json - Credentials JSON which will be used to initialize the GCP clients. we need a test to verify this authentication method works.

currently the GCP sanity tests use the credentials file located on the hosted machine, so we're only testing a single authentication option.

the test can be pretty simple - provide a a service account JSON and verify a client has been initialized successfully

Definition of done

• GCP assets inventory client initialization with WithCredentialsJSON is tested

Related issues

• https://github.com/elastic/cloudbeat/issues/2005