Update Provider Token Flow to OAuth2

[darnjo]

As part of the changes for the RESO Certification v4.0.0 release, we're migrating to using OAuth2 rather than API Keys.

The following changes will be required with this release:

• The sample.env and .env files should only have client_id, which will be the username, client_secret, which will be the current ApiKey, and token_uri, which will be the Cert API token endpoint that grants the bearer token to access services.reso.org.
• fetchProviderToken should change to use the _fetchClientCredentialsAccessToken method instead, which should be moved into a common library function and called by both Cert Utils for provider or admin functionality and during testing.