Support Microsoft SAML attribute names by default

What does this MR do and why?

Related to #420766 (closed)

Support Microsoft SAML attribute names by default

Add default attribute_statements to support Microsoft SAML attribute format. Microsoft uses a URL format by default. Prior to this change self-managed instances had to manually configure attribute statements and SaaS users had to add custom attributes to their SAML responses. This makes both self-managed and SaaS work out of the box with Microsoft.

Prior to this change we were merging defaults on top of user configuration. Now we need to make sure we merge user configuration on top of defaults so user configuration takes precedence. In the past this likely wasn't a big issue because defaults contained lower level configuration that GitLab needed to work properly with a given provider. But with the addition of SAML defaults, there's a strong likelihood users will override this configuration, and their configuration should take precedence. To give us a margin of safety since this behavior change could cause authentication problems for customers, I've included a feature flag so we can rollback the behavior if unforeseen consequences arise.

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