(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.
Would it be possible to explain why loading the custom oid file multiple times is bad?

We'd prefer not to track any class level state within `defaults.rb`, so we're hoping to remove that part of the code if possible.

Thanks!