idp_entity_id is redundant in Saml IdP creation payload

[luqmana]

We could simplify the SAML IdP creation payload a little bit. Today it looks like:

omicron/nexus/types/src/external_api/params.rs

Lines 551 to 582 in 21b31c8

	pub struct SamlIdentityProviderCreate {
	#[serde(flatten)]
	pub identity: IdentityMetadataCreateParams,
	/// the source of an identity provider metadata descriptor
	pub idp_metadata_source: IdpMetadataSource,
	/// idp's entity id
	pub idp_entity_id: String,
	/// sp's client id
	pub sp_client_id: String,
	/// service provider endpoint where the response will be sent
	pub acs_url: String,
	/// service provider endpoint where the idp should send log out requests
	pub slo_url: String,
	/// customer's technical contact for saml configuration
	pub technical_contact_email: String,
	/// request signing key pair
	#[serde(default)]
	#[serde(deserialize_with = "validate_key_pair")]
	pub signing_keypair: Option<DerEncodedKeyPair>,
	/// If set, SAML attributes with this name will be considered to denote a
	/// user's group membership, where the attribute value(s) should be a
	/// comma-separated list of group names.
	pub group_attribute_name: Option<String>,
	}

idp_entity_id is available as the entityID attribute on the root EntityDescriptor node from the IdP metadata XML.

[jmpesp]

It may also be possible to generate both acs_url and slo_url, though it may be better for those to be optional so they can be overridden.