Import nokogiri-xmlsec-instructure

[ccutrer]

Closes #2902

What problem is this PR intended to solve?

By having xmlsec as part of the core Nokogiri gem, we avoid the issues discussed in #2746. It also enables pre-compiled binary gems of xmlsec for all supported platforms by leveraging Nokogiri's pre-existing infrastructure for this.

Have you included adequate test coverage?

Yes.

Does this change affect the behavior of either the C or the Java implementations?

For now, this is C only.

[ccutrer]

At this point, all functionality works in CI (pending #3533). I do want to write some more tests, and improve the documentation. There is also still the issue that as-is this does not statically link OpenSSL, and depends on it being available already (as part of Ruby or from the OS). See discussion at #2902 (comment).