Build-time Secrets Injection

[evgeniy-scherbina]

The build-time secrets injection flow:

1. In provisiondserver in acquireJob function - retrieve secrets from the DB.
2. Extend AcquiredJob.provisioner.Metadata proto declaration with secrets.
3. Set retrieved secrets as AcquiredJob.provisioner.Metadata.
4. In provisioner/terraform update provisionEnv function to expose AcquiredJob.provisioner.Metadata Secrets as environment variables.
5. Later this environment variables will be read by data sources in terraform-provider-coder.