Plugin Agent Fingerprint Override

### Nomad version
Nomad v0.9.0-rc1 (7c00ab4f3f37cfd1e258b38fd2ad99e7bc23e4c3)

### Issue
While developing the nomad-driver-triton plugin (https://github.com/smithx10/nomad-driver-triton) I've stumbled upon the need to over-ride the Nomad Agents Fingerprinter. The triton plugin runs instances that are remote from the agent, and I'd like a method in the plugin that will return the agent's fingerprint. I imagine I'd query the Triton API to get Tenant Limits and populate the fingerprinter. For the time being, I've hacked around this issue by making every allocation very small.

### Reproduction steps
N/A.

Start by tracing Nomad's Agents Fingerprinter and plugin interface, then inspect the nomad-driver-triton plugin and its Triton API integration. Done means defining and implementing a supported way for the plugin to provide remote agent fingerprint data, with behavior validated against the allocation sizing use case described here.

Written by the indexing model from the issue text.