This adds new OIDC endpoints on the RPC endpoint. These two RPCs
handle generating the OIDC provider URL and then completing the
login by exchanging the provider token with an internal Nomad
token.
The RPC endpoints both do double forwarding. The initial forward
is to ensure we are talking to the regional leader; the second
then takes into account whether the auth method generates local or
global tokens. If it creates global tokens, we must then forward
onto the federated regional leader.
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