[nexus] Merge `SiloUserSessionList` and `ConsoleSessionList`

[david-crespo]

I added a SiloUserSessionList authz resource in #8479 in order to power the authz check on deleting all of a given user's sessions. Then I noticed we already have a ConsoleSessionList authz resource that we use to give the external authenticator user permission to create sessions on behalf of a user. I don't think we need to have both of these. Right now they're different in that the one I added references a particular user while the existing one is a global singleton, but there's no real need for the latter to be a singleton. It can just be absorbed into mine.

omicron/nexus/auth/src/authz/api_resources.rs

Lines 286 to 331 in a45cd1b

	/// ConsoleSessionList is a synthetic resource used for modeling who has access
	/// to create sessions.
	#[derive(Clone, Copy, Debug, Eq, PartialEq)]
	pub struct ConsoleSessionList;
	pub const CONSOLE_SESSION_LIST: ConsoleSessionList = ConsoleSessionList {};
	impl oso::PolarClass for ConsoleSessionList {
	fn get_polar_class_builder() -> oso::ClassBuilder<Self> {
	// Roles are not directly attached to ConsoleSessionList.
	oso::Class::builder()
	.with_equality_check()
	.add_method(
	"has_role",
	|_: &ConsoleSessionList,
	_actor: AuthenticatedActor,
	_role: String| false,
	)
	.add_attribute_getter("fleet", |_| FLEET)
	}
	}
	impl AuthorizedResource for ConsoleSessionList {
	fn load_roles<'fut>(
	&'fut self,
	opctx: &'fut OpContext,
	authn: &'fut authn::Context,
	roleset: &'fut mut RoleSet,
	) -> futures::future::BoxFuture<'fut, Result<(), Error>> {
	load_roles_for_resource_tree(&FLEET, opctx, authn, roleset).boxed()
	}
	fn on_unauthorized(
	&self,
	_: &Authz,
	error: Error,
	_: AnyActor,
	_: Action,
	) -> Error {
	error
	}
	fn polar_class(&self) -> oso::Class {
	Self::get_polar_class()
	}
	}