(PUP-2995) Proposal for processing an external trusted OID yaml file

* This patch is a proposal(that's why filename is hard-coded) that allows trusted facts users to map their
custom OIDs to user-friendly names that can be used in puppet manifests.

For instance a mapping file such as

```
# /etc/puppet/trusted_oid_mapping.yaml
---
oid_mapping:
  - ['1.3.6.1.4.1.34380.1.2.1.1', 'shortname', 'Long name']
  - ['1.3.6.1.4.1.34380.1.2.1.2', 'othershortname', 'Other Long name']
```

could be used to obtain `$trusted[extensions][shortname]`.


* If the oid mapping is not done at this level, each site that wants to
use custom trusted facts extensions would have to write its own
`oid_to_name` puppet function that traverse the `$trusted[extensions]`
hash and replace _OIDs_ with friendly names.

_Note_: I couldn't succeed to add a configuration option for this as the
`puppet/ssl/oids` library seems to be loaded very early and I don't have much time to dig deeper.
Can `mapping.nil?` ever be true? Or can `YAML.load_file` return nil and not raise?