Feature: Add new package providermeta

[MovieStoreGuy]

Context

Extends: #505

I wanted an easier way to provide the following:

• Keep the existing behaviour for configuring the provider
• Make it easier to test
• Make it easier to add unit tests with locally mocked endpoints and functionality.

The new package moves all the logic that currently exists in signalfx/provider.go into the new package which makes it possible use within unit tests without causing circular import issues.

Changes

• Moved Provider configuration into its own type MetaProviderLookup
  • These functions are okay to fail since they were originally intended to inject the required provider information without hard coding these into the tests.
• Added lookup functions for FileMetaLookup, UserMetaLookup, and NetrcMetaLookup to align with the existing configuration process.
• Helper functions LoadClient and LoadApplicationURL to reduce repeated code across the different resources.

[atoulme]

Please resolve conflicts. never mind, I merged the conflicts.