Investigate custom trust store for S3 gateways certificates for PBM

[Mehdi-Bendriss]

[Gu1nness]

PBM use the system certificates, you cannot specify a certificate to pick from what I know.
So to use a local trust stor" for the snap would mean:
* Update snap to create /var/snap/charmed-mongodb/current/etc/ssl/certs + prepopulate that directory.
* In the snap, add an environment variable to PBM and PBM agent that specify to look for certificates in /var/snap/charmed-mongodb/current/etc/ssl/certs.
* Update the calls to /update-ca-certificates to add --etccertsdir /var/snap/charmed-mongodb/current/etc/ssl/certs
* Optionally also create/var/snap/charmed-mongodb/current/usr/local/share/ca-certificates and same add the right argument to the update-ca-certificates call