Remove carbon registry usages in IS

[darshanasbg]

Is your feature request related to a problem? Please describe.
The registry is a content store and a metadata repository available in the carbon platform. These contents are keyed by unique paths where a path is similar to a Unix file path. And the registry provides additional features like grouping resources into collections, versioning, rating resources etc.

In Identity Server there are several features that use the registry as the data repository, such as Email templates, SAML configurations, XACML policies etc.. When using registry as the data repository in these features, there are limitations like,

• The (keyed by unique paths) data model is not optimal to store the data needed for these features
  -> which results performance issues
  -> tendency to compromise the data models in the feature development, in order to match the repository requirements
• Accessing registry needs loading the tenant which leads to higher memory use which limits horizontal scalability of the Identity Server

Also above IAM features don’t have a need to use additional features (like resource collection, versioning, rating etc.) that registry provides.

Also having registry core itself leads to operational complexities like,

• Additional 3rd party dependencies to keep updated with
• Issues in the registry core when handling higher number of tenants
• Additional image size, startup time, memory it brings to the product

Describe the solution you would prefer
Therefore it's needed to remove the registry usages of the product first and then remove the registry core.

In terms of the overall plan, there are two parts on solving this problem

• Remove registry usages from the features used in IS
• Removing the registry core features

Remove registry usages from the features used in IS

The below features will be removed using the registry.

• Carbon registry usage removal from the Email/SMS templates #19999
• Carbon registry usage removal from SAML #20992
• IDP SAML Metadata
• Remove registry usage from Key Stores Management in IS #21206
• Carbon registry usage removal from the XACML #20000
• Challenge Questions
• Admin advisory banner

Apart from above several features use registry as a dependency for some utility functions (not as a storage). These usages are tracked to removed with: #16162

Additional context