Support the Vault API for retrieving certificates

As an alternative to reading the `.pem` from the local filesystem, allow the use of Vault's [Read Certificate](https://www.vaultproject.io/api/secret/pki/index.html#read-certificate) call to retrieve the certificate PEM at packager startup.

Start by tracing the packager startup path that currently reads the certificate PEM from the local filesystem. Read Vault's linked Read Certificate API documentation and identify how its returned PEM should fit that path. Done means startup can retrieve the certificate through Vault while local filesystem loading remains available as an alternative.

Written by the indexing model from the issue text.