Add support for DeviceAuthURL and a Provider.Config() API
The go-oidc library added support for DeviceAuthURL in coreos/go-oidc#365. This adds it also to the cap library and also makes a ProviderConfig structure available through a new API call Provider.Config() so an application like the one added in hashicorp/vault-plugin-auth-jwt#131 that needs those URLs does not need to discover them itself. Note that it does not include JWKSURL because that is not available through the go-oidc API.
At the same time, this adds DeviceAuthURL to the DiscoveryInfo API.
- Replaces #139
@DrDaveD I'll try to get you some review feedback by end of this week; baring unforeseen circumstances.
@jimlambrt I suppose those unforeseen circumstances occurred? Can you get that review feedback soon?