Support configuration of localEntityIdTemplate for a SAML Relying Party

[picpicpic]

Add localEntityIdTemplate in saml2 autoconfiguration.

Current configuration

    saml2:
      relyingparty:
        registration:
          mysso:
            signing:
              credentials:
              - certificate-location: classpath:my-public.cer
                private-key-location: classpath:my-private.cer
            identityprovider:
              entity-id: entity-idp
              sso-url: https://mysso/public/saml2sso
              verification:
                credentials:
                - certificate-location: classpath:saml-signing-cert.pem

Expected configuration

    saml2:
      relyingparty:
        registration:
          mysso:
            signing:
              credentials:
              - certificate-location: classpath:my-public.cer
                private-key-location: classpath:my-private.cer
            identityprovider:
              entity-id: entity-idp
              sso-url: https://mysso/public/saml2sso
              verification:
                credentials:
                - certificate-location: classpath:saml-signing-cert.pem
            spprovider:
              local-entity-id: mylocalentityid

The 2 last lines allows to configure the local-entity-id.

Proposal

Add an internal class Spprovider in : org.springframework.boot.autoconfigure.security.saml2.Saml2RelyingPartyProperties
Use builder in : org.springframework.boot.autoconfigure.security.saml2.Saml2RelyingPartyRegistrationConfiguration

If needed, I can provide PR

Thx!

[mbhave]

@picpicpic A PR would be most welcome. I'm not sure if the Spprovider class is required since the signing credentials related to a service provider are at the top-level under Registration.

[wilkinsona]

Closing in favour of #22258.