Add ProviderName attribute to outbound SAML authentication request messages

[jcgarciaa]

Is your suggestion related to an experience ? Please describe.

When a federated SAML IDP expects to receive the ProviderName attribute on the AuthnRequest message, the authentication fails due to the absence of this attribute.

Describe the improvement

Although, the SAML2 specification [1] defines the ProviderName attribute as optional, WSO2IS should be able to set this attribute (from the existent properties of the federated IDP -or a new one-) when building the authentication request message [2].

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[1] https://docs.oasis-open.org/security/saml/v2.0/saml-core-2.0-os.pdf
[2] https://github.com/wso2-extensions/identity-outbound-auth-samlsso/blob/master/components/org.wso2.carbon.identity.application.authenticator.samlsso/src/main/java/org/wso2/carbon/identity/application/authenticator/samlsso/manager/DefaultSAML2SSOManager.java#L740