Add Jackson Support for saml2 Module

[ugrave]

This PR does the following:

• Adds a new Jackson Module org.springframework.security.saml2.jackson2.Saml2Jackson2Module which contains mixins for SAML2 classes
• Add new mixin for Collections.modifiableMap to org.springframework.security.jackson2.CoreJackson2Module

Closes gh-10905

[ugrave]

I have added a org.springframework.security.jackson2.UnmodifiableMapMixin to the CoreJackson2Module. Now i see there is already a mixin for UnmodifiableMaps bur this is placed in the OAuth2ClientJackson2Module (org.springframework.security.oauth2.client.jackson2.UnmodifiableMapMixin)
Question is how sould i handle it. Move/Copy the mixing from the OAuth2 module to the general jackson module and replace my version.

[jzheaux]

Thanks, @ugrave! I appreciate your thoroughness in the PR, I've left some feedback inline.

In preparation for merging, will you please squash your commits?

[jzheaux]

Thanks, @ugrave!

This is merged via df84826. I also added a polish of 6c3d183 and 931fb6a to remove some duplication and remove the authenticated attribute from Saml2AuthenticationMixin.