Extracts Saml::XML into SamlIdp::XML to free up the use of the Saml namespace

[pelted]

What this PR does:

• moves XML module out of the Saml namespace and into SamlIdp [^1]
• updates references from Saml::XML to SamlIdp::XML

Why

Defining such a generic namespace as Saml in this gem causes namespace collisions for projects attempting to use saml-idp which may define their own Saml module. We shouldn't do that here. There was even a note to that affect in the form of a long standing TODO item to extract this.

[^1]: This is opinionated to avoid the repetition of SamlIdp::Saml::Xml