Support Specifying Identifier for relying-party-registrations Element

[jzheaux]

When using <relying-party-registrations> to configure a RelyingPartyRegistrationRepository, it would be helpful to be able to give it an identifier so that it can be referred to other custom SAML components like so:

<relying-party-registrations id="ref">
    <relying-party-registration ..../>
    <asserting-party ... />
</relying-party-registrations>

<b:bean class="org.springframework.security.saml2.provider.service.web.authentication.OpenSaml4AuthenticationRequestResolver">
    <b:constructor-arg ref="ref"/>
    <b:property ref="relayStateResolver"/>
</b:bean>

...

This would involve updating RelyingPartyRegistrationsBeanDefinitionParser, its test file, and the accompanying .rnc file.

Related to gh-14417

[abhishek-bafna-amdhan]

Hello @jzheaux. I noticed another issue that can be looked along with this issue (#14645)

Both these issues may require changes in RelyingPartyRegistrationsBeanDefinitionParser. Please do have a look into the issue and provide your valuable feedback.