Read authToken from environment variables

[arabian9ts]

I am running Grafana on Cloud Run and installing the Sentry datasource using provisioning install. In this case, it is necessary to specify the authToken in secureJsonData, and it seems that there is no setting to read from environment variables.

When running Grafana in a serverless environment, I believe it is a common choice to embed the authToken from the secret manager into environment variables. Therefore, I would like to add an implementation that allows specifying the name of the environment variable.

For example, I would like to add an implementation that makes the following settings possible:

apiVersion: 1
datasources:
  - name: Sentry
    type: grafana-sentry-datasource
    access: proxy
    orgId: 1
    version: 1
    editable: false
    jsonData:
      url: https://sentry.io
      orgSlug: xxxxxxxxxxxxx
    secureJsonData:
      authTokenEnv: ENV_NAME_SENTRY_AUTH_TOKEN

if authToken, ok := s.DecryptedSecureJSONData["authToken"]; ok {
	config.authToken = authToken
}
if authTokenEnv, ok := s.DecryptedSecureJSONData["authTokenEnv"]; ok {
	config.authToken = os.Getenv(authTokenEnv)
}