Euclid: feature additions

[jagan-jaya]

• Add support to handle default fallback in the decision engine. Currently HS stores the default fallback in the configs table. But DE should handle this as a rule in the routing_algorithm table
• Support to add 3DS authentication rules in DE. Currently added the same in HS but not live yet.
• Create a doc for scenarios in which all routing flows are triggered.
• Metadata name refactoring, to process any key given for (metadata_variant).