feat: exchange Metal API keys for LB API OAuth2 tokens

[ctreatma]

This adds an oauth2.TokenSource that uses the token exchange endpoint to request an OAuth2 JWT for the given API key.

The TokenSource is added to the context under the lbaas.ContextOAuth2 context key. The load balancer API client automatically looks for a TokenSource under that key, uses it to get the OAuth2 token, and adds the token to outgoing requests.

[displague]

Here's a handy list of similar Apache 2 code in other providers where a managed loadbalancer service is handled:

• https://github.com/linode/linode-cloud-controller-manager/blob/master/cloud/linode/loadbalancers.go
• https://github.com/digitalocean/digitalocean-cloud-controller-manager/blob/master/cloud-controller-manager/do/loadbalancers.go
• https://github.com/oracle/oci-cloud-controller-manager/blob/master/pkg/cloudprovider/providers/oci/load_balancer.go
• https://github.com/hetznercloud/hcloud-cloud-controller-manager/blob/main/hcloud/load_balancers.go
• https://github.com/scaleway/scaleway-cloud-controller-manager/blob/master/scaleway/loadbalancers.go
• https://github.com/kubernetes-sigs/cloud-provider-azure/blob/master/pkg/provider/azure_loadbalancer.go#L184
• https://github.com/kubernetes-sigs/cloud-provider-huaweicloud/blob/master/pkg/cloudprovider/huaweicloud/sharedloadbalancer.go#L140

[ctreatma]

• ID of the load balancer
• Name of the load balancer
• Metro of the load balancer
• IP address of the load balancer
• Listener port that this service is using