OIDC Credentials do not allow multiple connections

[aeneasr]

Is your feature request related to a problem? Please describe.

At the moment,oidc.CredentialsConfig looks like this

type CredentialsConfig struct {
	Subject  string `json:"subject"`
	Provider string `json:"provider"`
}

and the credentials type is created as such:

	i.SetCredentials(s.ID(), identity.Credentials{
		ID:          s.ID(),
		Identifiers: []string{uid(provider.Config().ID, claims.Subject)},
		Config:      b.Bytes(),
	})

The problem with this is that while we might support multiple identifiers for the "oidc" credentials type, this would not reflect in the config portion at it is not an array.

Describe the solution you'd like

Instead, an array of providers should be possible in the config to support this use case. Additionally, we might add further things such as "AccessToken" and "RefreshToken" in there.

Describe alternatives you've considered

None.

Additional context

This is already documented as an array.